GnuTLS support is still broken in charybdis. On irc.indymedia.org, we couldn't link servers with gnutls - we'd get all sorts of errors like "Read error: Error in the pull function". Our experience was documented here:
https://we.riseup.net/ircd/outage-20170202 So I reopened this issue and marked unstable as affected. An upcoming upload should fix the issue by switching to the mbedtls backend, which behaved better (so far) in our tests. There's code in the debian/rules file to switch backends, as documented in debian/README.source. A.