caused by Chrome 80+ no longer allowing to send a synchronious ajax request from beforeunload handler, using sendBeacon (async request with keepalive=true) instead
server sends HTTP status "400 Bad Request" with JSON payload with "error" and "errno" attributes.
error is json_last_error_msg() prefixed with "JSON ".
Not yet implemented is resending the request (max. twice) for JSON parsing errors to try to work around network problems
Before we queried page_generation_time common preference without a callback, which send a synchronious request to server to fetch common preferences, when it returned it looked for the preference again and send the next synchronious request to the server, in an infinit loop. Now setting _callback parameter of egw.preference() to false, just returns undefined, if common preferences are not yet loaded, but dont try to query them from server
