Seconding this issue. Worked absolutely fine for years, including on these two very machines. For a while now it hasn't, and today I tried to fix it, thorough troubleshooting, reinstalling entirely, nothing.
Windows 11 primary won't even open settings from the taskbar when clicking "Configure Multiplicity." Nothing.
For true connection issues, I will bet a paycheck you will eventually find the issue working that connection page... In my 11 years, I have never seen it not resolve a 'all of a sudden it does not connect' report.
If it wont open the config at all, you have something else entirely going on... I would suggest a cleanboot...
https://support.stardock.com/space/SHC/1346601039/Multiplicity+Clean+Boot
Sean Drohan
Stardock Product Lifecycle Manager