I don't have a working existing board, it's a brand new installation. The database permissions are fine as I have installed two other forum software system, just to see if everything is fine with that.

So you installed two other ubb6.5.2 forum boards on the same server?
If you installed say other forum software that would have nothing to do with it. since I have no idea what privileges are required for the other software.

No I installed two different message board systems. But it shows that permissions with the database are correct. I also checked with my host.

For ubb the user will need all permissions usually.

Then the only thing I can think of other that trying again from scratch is since the version you have is like from 2006 is it is very old and may not compatible with the newer versions of mysql or php you are using.

It seems that ubbthreads version 6.5.2 was corrupt, because the support has just sent me version 6.5.5 and now I succeeded. I vaguley remember that I had the same problem four years ago when I installed the system back then... but I'm not 100% sure.

Good.
6.5.5 is jan 2006 so your version was prior and have no docs on that version so it could have been a bug on that version.
Probably due to newer versions of php/mysql or apache.

Yes, I'm really glad that it is working now.

If the message board is successful and gets as much feedback as I hope it will get, I will of course upgrade to UBBthreads 7, but I'd like to test it first before spending the extra money.

I'm setting up the forums now and everything seems to be fine.