Well, I know better now, thanx to Jesper Swerts-Knudsen (the father of a couple of howto's (sme.swerts-knudsen.dk)).
If you are reading this, you probably didn't get your particular problem solved even after following all hints above.
Well then, read on . . .
My users were climbing the walls (when they weren't busy beating me up), but luckily the solution was simple, once I got the hang of the underlying problem.
Here follows . .
After you have done the setprop thing, you might want to check ownership on the folders and files for each user.
In my case all the existing mailfiles were all right, only the mailfolders had changed ownership and were now belonging to root and admin respectively.
Bumber, right ?
Simply chown the lot, and enjoy your webmail (and perhaps another pint, in the spirit of Linus).