A client at CiviHosting who runs MediaWiki recently ran into errors with VisualEditor. We then discovered this error in their error log:

[23-Dec-2019 04:13:48 Europe/Sofia] PHP Warning: fopen(/tmp/mw-UIDGenerator-UUID-128): failed to open stream: Permission denied in /home/username/www/www/includes/utils/UIDGenerator.php on line 445

out of the blue. We fixed this issue by manually defining this variable:

$wgTmpDirectory = '/home/username/private';

We are always here to help.