Thaks Ted and Marc for your help and detailed explanations. As I
mentiones Subscribing to the trash (and spam, for tagging mails as such)
group seems to have solved the issue: at the very least, it has not come
back after some hours using Gnus. So I'm happy with this. As I've got
little experience with Gnus IMAP interface, I just wanted to be sure
that I wasn't misconfiguring anything on the client side.

Thanks again for your help,
jao
-- 
"Light thinks it travels faster than anything but it is wrong. No matter
 how fast light travels it finds the darkness has always got there first,
 and is waiting for it."
  -Terry Pratchett, Reaper Man