ulysses9994573 is it an "oops something went wrong" 500 error (generated by Flarum) or generic "500 error" (generated by the webserver) ?
If the only error you get is the internal redirects issue, then it's likely a misconfiguration in apache configuration somewhere, maybe something to do with your http->https redirects or something like that.
Flarum's htaccess defines a single rewrite that shouldn't cause any issue by itself. No change should be required to Flarum's htaccess file.