Error 400: "no_available_users_found_error" (Outlook Calendar)

[evolve-edge]

Hello everyone,

I’m currently facing a recurring issue when trying to create bookings via the Cal.com API using Make.com.

I’ve successfully connected my Microsoft Outlook calendar to my Cal.com account. However, every time I try to create a booking through the HTTP module in Make.com, I get the following error:

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Error 400: "no_available_users_found_error"
What’s interesting is that the exact same setup works flawlessly when using a Google Calendar — the problem only appears when using Outlook.

Has anyone experienced this before or found a workaround?
Would really appreciate any tips or guidance!

Thanks in advance 🙏
Marvin

[dosubot[bot]]

Hi @evolve-edge! I'm Dosu and I’m helping the cal.com team.

The "no_available_users_found_error" means Cal.com couldn't find an available user/host for the requested booking slot. This is often tied to how availability is checked across connected calendars. Outlook integration recently changed: it now uses caching and webhooks for real-time updates, which can affect how availability is detected compared to Google Calendar. If the cache isn't updating or the webhook isn't firing, Cal.com might think you're unavailable, even if your calendar looks open PR 21072 PR 21854 PR 22312.

Troubleshooting steps:

1. Double-check that your Outlook calendar is connected and selected as the primary calendar in Cal.com.
2. Make sure the calendar-cache feature flag is enabled (if your setup uses feature flags).
3. Try disconnecting and reconnecting your Outlook calendar to refresh credentials and webhook subscriptions.
4. Create, update, or delete an event in Outlook and see if Cal.com reflects the change—this tests if the webhook and cache are working.
5. If you have multiple calendars connected, try removing all but one to rule out conflicts (this has resolved similar errors with other calendar integrations see this issue).
6. Confirm that Cal.com has all required permissions for your Outlook account, especially for reading free/busy status.

These recent changes mean Outlook and Google Calendar may behave differently, so issues like this can appear only with Outlook. If the above steps don't help, let me know what you find at each step—there may be more targeted troubleshooting based on your results.

To reply, just mention @dosu.

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[evolve-edge]

Hey Mate,
the Problem ist the Calendar is working in Cal, the only problem is the https connection which is not working and giving me this failure, also when I check for spots in the calendar the https is working. I'm currently working with v1 should I switch to v2 for the booking, I also got recommend to try Cal.com Teams.

[dosubot[bot]]

Switching from API v1 to v2 or using Cal.com Teams probably won't resolve the "no_available_users_found_error" if the root cause is with Outlook integration. Both API versions and Teams use the same core availability logic, so if the calendar works in the UI but not via the API, it's likely due to Outlook-specific issues like caching, webhook sync lag, or permissions—not the API version or Teams logic.

Recent updates have fixed similar errors, especially in Teams, so make sure you're on the latest Cal.com version (see details). Try disconnecting and reconnecting your Outlook calendar, trigger a calendar event to test if the webhook/cache updates, and ensure only one calendar is connected to rule out conflicts. If the problem persists, enabling debug logging or capturing the full API error response may help pinpoint the issue.

To reply, just mention @dosu.

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[evolve-edge]

Hi Dosu,

Thanks again for your previous responses on GitHub. I've now gone through all the suggested steps, but unfortunately the issue still persists. Here's a complete overview of the situation:

🧩 Issue

When trying to create a booking via the Cal.com API (v1) using a Microsoft Outlook calendar, I always get the following error:

{
"message": "no_available_users_found_error",
"data": null
}

This only happens with Outlook. Using the exact same setup with Google Calendar, everything works perfectly through Make.com and the /v1/bookings HTTP request.

✅ What works fine

In the Cal.com UI, I can see all available and blocked times correctly.
The Outlook calendar is set as default.
The Outlook connection is active and syncing properly.
I can successfully retrieve available slots via the API using GET /v1/slots
GET /v1/users returns the correct user ID: 1625205
Creating, editing, or deleting events in Outlook correctly updates Cal.com availability.
The Event Type is properly configured (duration, calendar assignment, no booking limits).
The API request includes all required fields: start, end, eventTypeId, responses, metadata, etc.

🔁 Troubleshooting steps already done (all without success)

Reconnected Outlook calendar multiple times
Created events in Outlook → they appear in Cal.com
Ensured only one calendar is connected (Outlook)
API key passed correctly (tested via Header and Query param)
Azure permissions granted: Calendars.Read, Calendars.ReadWrite, offline_access, User.Read
GET /v1/slots returns correct availability slots
GET /v1/users returns valid user ID (e.g., ``1625205)
POST /v1/bookings using this user still returns no_available_users_found_error
I can't create a Team in Cal.com because that's only possible on paid plans. (Do I need a Paid Plan?)

❓ Suspected root cause

My suspicion is that the issue is related to Outlook-specific caching or routing logic within the API — even though the UI reflects everything correctly.

💬 Questions

Is there a way to manually trigger a cache/webhook refresh for Outlook?
Can I explicitly specify the user (via userId, routing.teamMemberIds, etc.) without using a Team?
I’m technically experienced and happy to provide logs, run specific tests, or enable debug output if that helps further diagnose the issue.

Thank you again for your time and support 🙏

[dosubot[bot]]

You can manually trigger a cache refresh for Outlook by calling the /api/calendar-cache/cron endpoint—this works for individual users as long as the calendar-cache and calendar-cache-serve feature flags are enabled; you do not need a paid Teams plan for this to work. Outlook calendar availability is kept in sync using Microsoft Graph webhooks, and the cache is updated automatically when events change, but the cron endpoint can force a refresh if needed (details).

For bookings, you can specify the userId directly in the API v1 request if you are a system-wide admin or organization admin; otherwise, it defaults to the authenticated user. There is no need for Teams or routing/teamMemberIds for individual bookings in v1 (code reference).

A paid Teams plan is not required for Outlook cache/webhook refresh or for individual bookings—these features work for individual users if the correct feature flags are enabled (source).

If the error persists even though availability is correct in the UI and via /v1/slots, it may be due to a subtle cache or sync issue. Try manually triggering the cache refresh as above, and double-check that only one calendar is connected and selected as default. If you can enable debug logging or capture the full API error response, that may help pinpoint the root cause.

To reply, just mention @dosu.

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[evolve-edge]

Hi Yosu,

Thanks again for your help and the detailed explanation earlier.
I just wanted to let you know that the issue with the no_available_users_found_error has now been resolved on my side.

It turned out that the problem was caused by an incorrect datetime format in the API request. I had initially passed the start parameter without seconds (e.g., 2025-07-31T13:30), but Cal.com expects a fully formatted ISO 8601 UTC timestamp including seconds and the "Z" suffix (e.g., 2025-07-31T13:30:00Z). After adjusting the format, the booking went through without any issues.

To clarify:
Availability was always correctly shown in the Cal.com UI.
/v1/slots also returned the expected open times.
The issue only occurred during POST /v1/bookings calls due to the missing seconds or incorrect time.
Now that I include the full timestamp in the correct format, everything works as expected — even without using Teams or additional routing fields.

Thanks again for pointing me in the right direction with the cache/webhook sync insights!

Best regards,
Marvin