I set up two collections for cross-collection searching, using these
lines in collect.cfg:

supercollection istb istbpub2
authenticatecollection
auth_groupistb_users

When I now click on either of the two collections's buttons, I always
get redirected to the login form. After logging in, I can perform the
desired action, but having to login repeatedly is a pain.

Any idea as to what might cause this?

Thanks,

Birgit