I noticed what the problem is.. Don't know how ot fix it though.
The phpbb forum is a add-on domain of my main website so if I visit
If I visit the website from https://siteurl.com/subdomain
> then I can move/delete topics correctly.
If I visit it from http://siteurl.com
> then I get that 500 error when moving/deleting topics..
What I don't understand is how to correct this problem. All the settings in phpbb point to the correct url being the actual domain and not the subdomain of the first domain.
Btw my first domain does have an SSL cert. the 2nd one does not