I didnt find anything in the flarums own logfiles, but your remark to look into the system log made me find something. The whole stack trace is 600 lines long and repeats itself over and over, the error messages above each 200 line stack are:
[Wed Aug 18 07:09:13.508717 2021] [php7:notice] [pid 2985] [client 127.0.0.1:44796] PHP Warning: Uncaught Error in exception handling during call to Error::__tostring() in /home/scumi/git/flarum/vendor/illuminate/collections/Traits/EnumeratesValues.php on line 920
[Wed Aug 18 07:09:13.517664 2021] [php7:notice] [pid 2985] [client 127.0.0.1:44796] PHP Warning: Uncaught Error: Xdebug has detected a possible infinite loop, and aborted your script with a stack depth of '200' frames in /home/scumi/git/flarum/vendor/illuminate/collections/Traits/EnumeratesValues.php:920\nStack trace:\n#0 {main}\n\n thrown in /home/scumi/git/flarum/vendor/illuminate/collections/Traits/EnumeratesValues.php on line 920
[Wed Aug 18 07:09:13.529629 2021] [php7:notice] [pid 2985] [client 127.0.0.1:44796] PHP Fatal error: __debuginfo() must return an array in /home/scumi/git/flarum/vendor/illuminate/collections/Traits/EnumeratesValues.php on line 920
Any convenient way to send you the log?