@KarstenV glad to see that the reinstall worked at least somewhat. I am surprised that Plex did not re-install, though good that you were able to do this yourself again. Did you see any specific errors why that failed in the rockstor log?
As for the other items:
check out this discussion thread:
The mentioned issue in that thread is still open, as it’s still not entirely clear why it happens (my attempts at reproducing it were only marginally successful then). So I think your best bet is to use the nmcli or nmtui to set a static address, if you can’t/don’t want to force it via static IP address on the dhcp router.
As for the ssh issue. I have not ecountered that before, are you using PuTTY (and if so, SSH Protocol version 2 setting, found under the SSH section) to connect or some other ssh client?
It seems that older versions of PuTTY can run into that issue related to which HostKeyAlgorithms are allowed/used.