It's a bit finicky at best. Try selecting each of your domain names in turn from the management drop-down in the Hosting Control Panel, then launching phpMyAdmin, and see if you have any luck launching from the other CGI domains. IIRC, you need to have the domain selected that you installed against.
If that doesn't work, then it might be worth you trying to uninstall/reinstall the service.
The only back-end change that I can think of is a fairly recent PHP upgrade, but I'm not aware of any widespread knock-on effects that's had on phpMyAdmin installs.
If you want a personal recommendation then I'd suggest you forego phpMyAdmin and use something like Adminer instead.
Bob Pullen Plusnet Product Team If I've been helpful then please give thanks ⤵