Hi @ivan.csaba ,
Is this issue affecting all users in your organization?
Any changes were made in the environment right before the error started around 1 month ago?
Based on my experience, please run the following command to check the authentication settings for OWA and Exchange ActiveSync:
Get-OwaVirtualDirectory -Server <ServerName> | fl *auth*
Get-ActiveSyncVirtualDirectory -Server <ServerName> | fl *auth*
Below is the output returned in my Exchange 2019 test lab, you may compare it with your result and see they look the same:
With the above confirmed, it's suggested to try resetting FormsAuthentication for OWA virtual directory and restarting IIS to check the result:
Get-OWAVirtualDirectory | Set-OWAVirtualDirectory -FormsAuthentication $False -BasicAuthentication $True
Get-OWAVirtualDirectory | Set-OWAVirtualDirectory -FormsAuthentication $True -BasicAuthentication $True
Followed by IISreset and also to recycle the app pools for OWA and ECP in IIS.
Reference: Invalid username and password accessing OWA and ECP
In case the above doesn't work, I'd recommend have a look at the Event Viewer on the Exchange server for any relevant events. Furthermore, you can go through the IIS logs which by default located at %SystemDrive%\inetpub\logs\LogFiles and see if any clues can be found.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.