I think this all seems to be correct, I stupidly emptied the database entry "ossnsitesetting/upgrade". This is probably the cause, unfortunately I did not make a backup.

On the modified files, take a look to check if there are some strings without slash. The string must be as this example

    'onesignal:notification:tip:state:subscribed' => "You\'re subscribed to notifications",

As other attempts to fix, check for error messages in developer tools and take a look in the error_log file.

Thanks, I can't do anything with JS, I mainly changed something in langugae files.
The website seems to work, except that it displays a blank page.
Where could I look first?
The deleted folder in ossn_data/memcached was created again and there are files in it.

If you changed some files with JS code and applied the format document command, check if the line with

//<script> 

has changed to

//
<script>

This occurs to me when I run the Format document command