7
I solved my specific problem (see above) myself by restoring the forum step by step. The tables of xphpbbi are (of course, stupid me) sometimes different from the original phpbb tables. So I restore what tables I can through PHPmyAdmin.
The users are a problem, but since there are only 15 of them right now, I'll just insert their data manually into the xoops_users table. That seems to work, for now ...
I don't know any SQL but you can get a long way by experimenting and improvising
Apart from all that, xphpbbi works just fine here. No bugs yet.