Are your absolute path settings all the same.
Meaning all using public-html or www?
Not a mix of one path is www and another path is public_html.
Note the double slash
www.domain.com//www.domain.comI had this issue with a previous host but not my current one.
So I believe it is some server configuration that screws with the ubb scripts.
My previous host would just say call the vendor.
Before I changed hosts I think the following fixed it.
But I can't recall it was three years ago.
https://htaccessbook.com/remove-double-forward-slashes/All I remember was it was a edit to the htaccess file.
When I changed hosts it worked fine without the fix but I never could figure out what was different between the the two hosts.
Because my account at the previous host was closed to quickly during the transition.