I think I can tell you exactly why the problem is occurring with IBrowse. When posting form data, IBrowse always terminates the form data in the HTTP request with a CRLF (AWeb does not). In this case, the amiga.org server does not bother to send a HTTP response back, hence the error message from IBrowse. Having changed IBrowse to not send the CRLF after the form data, the problem went away completely.
So, it seems that the xoops scripts do not handle this. Also, were there any changes to the server or php versions? Apparently, some servers don't bother passing the trailing CRLF onto the script anyway, so that could also explain why xoops had no problem before, but does now (assuming the xoops scripts haven't changed in the way they handle this.
I'm not 100% sure (yet) whether this should be made a permanent change in IBrowse - clearly, there must have been a reason for sending the CRLF for all these years :-) And it's quite possible IBrowse does because other browsers do the same (or did do in earlier versions).