Actually, the UBB.classic import script is for UBB.threads 7. It may need some minor adjustments for the 7.6/7.7 series
The included redirect script ultimatebb.php definitely need some changes after it still uses $HTTP_SERVER_VARS in the find_environmental function.
The importer script will work to import into UBB.threads 7.2, you'll need to upgrade from that build as UBB.threads v7.3 added the Global Moderator group which assigned itself the previous moderator group id; importing directly to the latest build will make all of your users moderators, and your moderators GMs.
Phillipp, are you representing Alento's forum? I'm confused why you just interjected comments about the redirect script into a discussion about importing UBBCGI to UBB.threads...
If Alento's forums are not currently up and running, and on a RECENT BUILD of UBB.Classic, the redirector scripts will not work as they will be assuming locations are from within the final series of the product. He'll likely not be using the redirect scripts at all as UBBCGI is not UBB.Classic, and certainly nowhere near the final builds of UBB.Classic (which the redirect scripts would assume).
If these forums are still up and running, do you have a location to them for myself or Isaac to take a look at? We'd need to know exact versions, and it sounds like you're talking about UBB.Classic, but you mention v1 which would have been UBBCGI.
The big thing here is, the import may just not work, and I myself wouldn't be comfortable working on a project that just outright may not work (as I charge for my time as a service) and the outcome from such an old relic may just be that you may need to make an archive (no new posts/registrations/logins allowed) and start new.
For the record, the path I'd take for upgrading would be: Ancient -> UBB.Classic v6.7 -> UBB.threads v7.2 -> UBB.threads Current; now, getting your ancient to a build capable of being imported to UBB.threads Current is a huge issue, depending on the actual version of whatever product, you'd still need to get to the v6.7 series to import to modern, so you'd likely have to see what builds of UBB.CGI would have to be imported to UBB.Classic (UBBCGI and UBB.Classic are different products, just like WWWThreads and UBB.threads are different products) then you can import Classic to Threads.
Also, UBB.Classic/UBBCGI are Perl scripts, and shouldn't really matter as far as PHP builds go, with the exception being the "UBB Accelerator" built into UBB.Classic; but UBB.C has been End of Life since UBB.threads7 was released on October 5th, 2006.