In the meantime I've found a workaround. Apparently it is a problem
with gnutls. If I kill the process:
killall gnutls-cli
and check the mails again, then it works fine, without restarting
emacs (hooray!). Sadly I don't have the time (and maybe not the brain)
to hack into gnus to see if this can be solved more elegantly.
But after all we live in an imperfect world, don't we?
Aha. that's surprising. But using "gnutls-cli" alone doesn't give similar
problem.Apparently, we'r not the only victims, and what Stefan suggested
sounds reasonable : "In any case what I see happen is that the
underlying connection is stuck and doesn't timeout and Gnus's imap backend
doesn't have its own timeout to make up for it."