After the upgrade I had similar problems:
- couldn't connect, or got quickly disconnected.

After a while (couple of hours), things stabilised and I was able to stay connected.
Then, no way to exchange message, everything I tried sending remained with a "red dot".
But once I asked a nearby friend to msg me (private message), everything back to normal. I received her message, and she received the few message which were wainting with a red dot.

So try to send a message to a friend and ask her/him to reply to it.

WhatsApp has made encryption mandatory, so either the message is needed to make sure that both end-points agree about encryption, or both decide to fall back to non encrypted communication.
(Is axoltl active in the latest whatsup build ?)

Similar situation with the groups:
- by asking people to send me individual messages, I sometime start again to receive them in group discussion.
(Though sometime, after a while it goes back to missing messages).

I've never ever needed to re-register since I have whatsup on my jolla.
(and similarily, never needed to re-register with mojoWhatsup on my WebOS Pre3, until I got definitely disconnected (but not banned) after some update from WhatsApp at a time where development of mojoWhatsup was already abandonned)