Okay, thank you, but last entry in MYBB error.log is 3 days old and was an MYSQL error because of an other plugin.
To be honest, I made it very easy. I simply renamed all files and then searched for "recentthread" and replaced it with "recentthread2" in all files at once with Notepad++. So it should not be a PHP syntax error. I checked the files, some "recentthreads" are renamed "recentthread2s", but it was the same execution for all files (replacing "recentthread" to "recentthread2") and I made the exact same thing with the files/folders (recentthread = recentthread2 and recentthreads = recentthread2s).
I thought this should work.
Please notice, the attached file above is not up to date. Here is the newest version with all changes I made, mentioned in this post.
So, I turned on PHP errors in the htaccess and this is what I get:
Fatal error: Cannot redeclare recenttthread_admin_tools_get_admin_log_action() (previously declared in **************/new/forumupdate/inc/plugins/recentthreads/hooks.php:663) in **************/new/forumupdate/inc/plugins/recentthread2s/hooks.php on line 663
I will check this line now...
Seems like a typo, because "recentthread" was written with 3x "t" in this function, so I renamed it to "recenttthread2_admin_tools_get_admin_log_action" now and the 500 is gone.