Okay, I did that and can now view the error directory.
In the sql_errors file I can see an entry saying (I removed the http part of the error lines)
[Referring Page] ..../adm/admloginuser.php?S=&SQ=0
?819 ?1161336543?(FUD/include/theme/default/db.inc:72
FUD/include/theme/default/users.inc:798
FUD/include/theme/default/users_reg.inc:267
..../forum/adm/admloginuser.php:22
) 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
Query: UPDATE fud26_users SET sq='4e2a692c8cfcc20b7a77f17bb911bf35' WHERE id=
_POST: array ( 'S' => '', 'SQ' => '0', 'login' => 'admin', 'passwd' => 'MyPassWd', 'btn_login' => 'Login', )
Server Version: 4.1.11-standard
[Referring Page] http://myforum/forum/adm/admloginuser.php?S=&SQ=0
Looking at the 1064 error for MySQL hints at some sort of version change between the backup and the new situation. I'm going ot investigate that, but if you see anything else that may give me a clue as to what's happening please let me know.
Cheers
Chris