Hey there @Adel86832 - thanks for reaching out on behalf of your client! Issues with calendar connections can be frustrating, and I’m sorry this is happening. You have taken quite a few of the steps we’d initially advise be taken - so let’s dive in a little deeper to try to resolve this.

Firstly - was his O365 Calendar working at any point? Did it work and then stop working, and if so, when was this error first noticed? Or, did the calendar connection never successfully work? 

Here is some info to keep in mind: 

• This is a web-based connection. Calendly connects to your Outlook calendar via Outlook/Office 365's API.
• Your IT or Security team will need to allow permissions to third-party apps before you can connect your calendar
• If your company does not want to allow this, they can alternatively grant permissions to only Calendly rather than all third-party apps

So, check out the below:

1.  Can your client login with the calendar credentials associated with their O365 Calendar at http://outlook.office365.com/ and view their calendar? If so, this is a good sign and means they have a web-based calendar.
2. If they are not able to login with their credentials at that website, they do not have a web-based calendar and might need to use our Plug-in - read more here and here. 
3. Alternatively, you mentioned Exchange. This is a different calendar connection than the O365 Calendar connection and might be causing the issue (meaning the wrong calendar connection might have been selected when attempting to connect on the Calendar Connection page in your client’s Calendly account). If this is the case, please read here. 

If after going over all of this you are not able to resolve the issue, you’ll likely benefit from troubleshooting on a more personal level with our support team. They can be reached 24/7 by logging into your Calendly account (this will be best done from your client’s account) and clicking “help” where our live chat support will prompt you to answer a few questions and then connect you with a live agent. 

I hope this helps! Let me know if I can further assist, as well!

. 

Hello Kelsi,

I appreciate your swift reply.

We have assigned a Microsoft 365 E5 license for him. His calendar functions properly on both the Outlook desktop application and OWA. Additionally, we have configured SSO for their enterprise application through their Microsoft tenant, and I have ensured he has the necessary permissions to utilize Calendly.

Other users in his office are not encountering any difficulties with accessing and utilizing Calendly. They're all included in the calendly SSO application. 

It appears to be an isolated issue specific to this user.

Hey there @Adel86832 - thanks so much for this information. Due to the fact that the user seems to be isolated in the unexpected behavior, other Calendly users in your organization are not encountering the same issues, and you’re clearly well educated in how this works and I cannot take troubleshooting further via community - I will need to ask that you do reach out to our support team as outlined above. You might also need to reach out to Microsoft’s support team, but I do suggest starting with ours! They are available 24/7 (including now!) and can be reached via live chat by logging into Calendly and clicking “help.” I’m sorry that I cannot be of more assistance from my end of things!