Using Classic Outlook on Windows for personal email, calendar, and contact management
Hello Galambos Róbert,
May I confirm if you add your Office 365 account as IMAP type into Gmail and Thunderbird for Android client? If so, I would suggest you try remove your account from those clients and then re-add it back to check the results.
Note: Please back up your data before you remove it.
Meanwhile, to rule out mobile client problems, if it is convenience, please add your account into desktop client or Outlook client see if it can help.
If the problem persists, would you mind provide us a screenshot of the error message to help us understand your symptom more clearly?
Best Regards,
Sylvie