actually i discoverede a similar error installing the UBB on a linus machine.
In quite a lot of the .cgi files you will see an occurrence of
print ("content type: html/text/n/n");
on the machine in question, this caused en internal server error message when trying to run the script.
If you manually edit the line, and removes the () (but remember to keep a space after the final ") the server will be able to run the script.
at least it's worth a try.