gnus: how to restore the active file

[Francis Moreau]

Hello,

I currently use nnfolder backend for my emails.

I tried to create a new group using the nnml back end but it seems
that my previous active file (stored in Mail/active) has been screwed.

Now I can access to my nnfolder groups anymore: when I try to enter to
a group I get:

Couldn't activate group nnfolder:save.divers.comp: No such group:
save.divers.comp

Does anybody know how I can restore it so that can reuse my nnfolder
groups ?

Does it means that I can't have several groups using different back
ends ?

Thanks