After updating and restarting Plex I get this error.
DESKTOP-JS1T7UP is currently unavailable.(my desktop computer)
I click on MainServer and everything is fine. Why does it try to connect to desktop and is there a way to prevent it from happening again.
There are still older Plex servers connected to your account.
If you only use one plex server, you should remove those older, now probably obsolete servers from your account.
So the various Plex clients don’t try to reach them anymore.
If some of your clients have their default server still set to one of the obsolete servers, you may also have to reset this preference as well: https://support.plex.tv/articles/customizing-plex-web/
(scroll down. ‘Reset Customization’)