I solved the 404 internal server error. It required starting out with a clean install of J1.7.3 from the J1.6.5 that couldn't be upgraded without producing errors. I had tried doing a SQL import into the J1.7.3 but that didn't work too well. I ended up doing a complete deleting of all file and starting with a clean install with the newest versions of all the major software - j1.7.3 and CB1.7.1, Groupjive, kunena forums, acymailing.
I then imported the old members data SQL file into the clean install. I still had to manually rebuild the site content....
Anyway, the problem is now solved! Yaaaaa