Strange Error on Plugin Install and Update

[mneumann]

When I try to install a plugin from the repository, I get the error: Installation failed: Download failed. Unauthorized
When I look though at the installed plugins, it is there and working perfectly.

The same with updates.
When I try to update a plugin, I get the error: Update failed: Download failed. Unauthorized
But the update gets applied anyways.

So it appears it is working fine, just getting an error message that should not appear. Anybody has an idea what could be the issue?

 

[mneumann]

Found another hint in the error log:

[STDERR] PHP Warning: Undefined property: stdClass::$plugin in /var/www/vhosts/staging/html/wp-includes/class-wp-list-util.php on line 168