Inka, if you don't hack ubb_forum_summary.cgi and ubb_forum, it won't work with cache.
Try the following instead: (untested)
Use IM hack's ubb_lib_files.cgi instructions, but in 'add below', add the following:
$name = &ProcessVIEWS($name); $name is $ThisHTML, $thisvar or $str, depending on what's on IM hack.