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.com
I 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.


Blue Man Group
There is no such thing as stupid questions. Just stupid answers