9090 is the port where our so-called metasfresh-admin would run if it was started. The different metasfresh services try to connect to it once a minute. If it's not running, nothing bad happens besides that log message.
If your update to 5.10 went well after the workaround, you should be able to start metasfresh-admin via
systemctl start metasfresh-admin.service
the different metasfresh services should then report in and you can check out some of their internals by pointing your browser to port 9090.
PS: I'm not at my desk so the
systemctl .. might have typos again.