Looks like the share-session started successfully (it showed "node-1.indihub.io:53023" public endpoint which can be used by guest in KStars/Ekos).
According to "API-server error: listen tcp :2020: bind: address already in use" error
- looks like agent was already running in a background or in other terminal? indihub-agent starts server on a port 2020 and that port was already taken.
Try command "ps -ef | grep indihub-agent" to see if it is already running (i.e. it could be launched from INDI Web Manager).
If you are sure that no other copy if indihub-agent is running that would mean some other program on stellarmate takes that port. Not a big deal though, you can use "-api-port=N" parameter where N is some other free port, i.e. 8787 or something.