for anyone stumbling over this:
the server process did not start, found this out after cheking the logs:
journalctl -fu snap.rocketchat-server.rocketchat-server
the i saw that the db was faulty and found this: Rocket.chat not starting after SNAP version refresh 5.4.5 -> 6.0.0 - #14 by arin
i then did this: Rocket.chat not starting after SNAP version refresh 5.4.5 -> 6.0.0 - #17 by stefan.l
thanks and byebye