@etienne hello! I'm not an expert, but one thing to check:
Does this problem happen only for existing messages, or also for new messages sent while Converse is running?
Reason I ask: when a message is encrypted with OMEMO, it is only encrypted for the devices that already exist.
This has some nice security benefits but also means when you set up a new client, it can't decrypt the old messages.
It "should work" with new messages - so if it doesn't, that suggests an incompatibility or bug 🙂