Since this this workaround is under different thread and thus hard to find, it makes sense to have higher visibility here as well.
Problem: XMPP connection fails to be established after upgrade to Sailfish OS 4.0.x
Symptoms: Connection fails silently in UI. If you have access to server logs, then you may see generic connection errors in logs (e.g. for ejabberd “Failed to secure c2s connection: TLS failed: SSL_do_handshake failed: error:somenumber:SSL routines:tls_early_post_process_client_hello:unsupported protocol”)
Solution: As per @jpa advice here Missing support for TLS1.3 replace original telepathy-gabble (pkcon remove telepathy-gabble) and install the telepathy-gabble-plus package from openrepos.
Future: This bug is picked up by Jolla devs and may get fixed in future releases and you may want to revert to original state later. Also removing of telepathy-gabble-plus may be required before next bigger upgrade.
Sidenote: telepathy-gabble-plus contains fixes, that would be useful systemwide and I wonder if it is possible to incorporate this lib into mainline?