Lars Magne Ingebrigtsen <larsi@gnus.org> writes:

>> My understanding is that levels are per-group and not per-server, and
>> all groups defined in ~/.newsrc.eld have level 3.  So I'm wondering
>> which group Gnus is trying to open.
>
> Could you try to `M-x debug-on-entry RET nntp-open-server RET'?  The
> backtrace should then possibly tell you what group is triggering this.

Sure, please find below the backtrace.  I did (1) load gnus-int.el and
nnimap.el, (2) M-x debug-on-entry RET nnimap-open-server RET, and M-x
gnus-no-server.  There seems no specific group appeared in the log.