In OSSN 5.1 the views of 'Message' and 'Notifications' show white pages with no functions.
Deacitvation of the Message-Component in the Admin-account crashes the whole site (HTTP 500 ERROR)
The Installation-Routine didn't show any errors at all. We hadn't that issues in versions before.
The problem was solved by a fresh installation, using a subdomain (!!) instead a folder during the installation process.
I used 'ossn.mydomain.tld' instead of 'mydomain.tld/ossn'
Allthough ossn does not require this, it has helped.
The issue didn't appear before and didn't generate any error logs. The html of the 'message' and 'notification' site were thrown out, but not displayed. The only reason this could have, was a file that got damaged while uploading.
500 error means there is something wrong with the files or undefined stuff (500 error didn't shows exact error, there must be something in your error_log file). Did you followed the upgrade instructions mentioned on our website? Can you try to follow https://www.opensource-socialnetwork.org/wiki/view/1101/how-to-report-a-bug-error ?
Did you tried to install v5.1 separate on same hosting for testing ?