Error: No Mailbox And No License Assigned Exception

I noticed that you use English in your messages even though it is an Finnish-speaking forum; I will therefore continue in English so that my answer is as clear and useful as possible. If you'd prefer me to switch to Finnish, please let me know and I'll gladly do so. Also, since our test environment uses the English interface, the screenshots will appear in English. Thank you for your understanding!

Dear @EOVS,

Good day! Welcome to Microsoft Q&A forum!Based on your description, I understand that you are unable to access the mailbox for your account. When attempting to sign in to Outlook, you receive the error "OwaUserHasNoMailboxAndNoLicenseAssignedException" (Error 500), which suggests that the account may no longer have an Exchange Online mailbox license assigned. You are looking for assistance verifying and restoring the license so that mailbox access can be regained.

May I ask if you are an end user or an admin of your organization’s Microsoft 365 tenant? This will determine what steps you’re able to perform on your side.

If you are an end user, please kindly ask your IT administrator to check your account in the Microsoft 365 Admin Center. Then go to Users > Active users, select your username, then open Licenses and apps tab. Under the Apps section, please make sure that Exchange Online (Plan 1 or Plan 2) is enabled.

If the license is already assigned, the administrator may try removing and reassigning it to the affected account. After making the change, allow some time for Microsoft 365 to update the mailbox configuration. This can help refresh the mailbox provisioning process and may resolve the error.

If you do not have administrative access, I recommend contacting your organization's Microsoft 365 administrator, as they will be able to verify the license assignment and mailbox status on the account.

For additional guidance, please refer to the following article: Getting a mailbox not found error in Outlook on the web - Microsoft 365 admin | Microsoft Learn 

I hope this information is helpful. Please follow these steps and let me know if it works for you.

Kindly let me know when there are updates or if you need further assistance. Any updates you’re able to share would be really helpful. I appreciate your time and look forward to hearing how things are going! 

Thank you for your time and patience.