Warning: preg_match(): Allocation of JIT memory failed, PCRE JIT will be disabled. This is likely caused by security restrictions.
Either grant PHP permission to allocate executable memory, or set pcre.jit=0 in /home/user/domain.xxx/FUDforum/include/adm_common.inc on line 24

Looking into this for you, I found that pcre.jit=0 was not set in your phprc file (user/.php/7.4/phprc). This error is a bug with PCRE
To work around this bug, We recommend adding the following line to your site's phprc file. pcre.jit = 0
This setting accommodates the majority of all PHP websites. However, if you specifically need this enabled, you can adjust the setting to the value of 1 in your phprc file. I went ahead and added that and our recommended PHP limits to the file for your site user. The phprc file will need to be updated for the site user of any other site you see this error on any other sites in the future.