Parallels 20 & 26 hangs with "starting..." message and/or repeated requests to reboot

I have a 2025 Macbook Pro M4 Max running Sequoia current version. I have tried starting Parallels 20 and by mistake, bought Parallels 26 but neither will get past the starting... splash screen. I get the same behaviour when clicking on the virtual machine or the app icon. The screen appears for a minute or so and then quits without any warning other than the occasional message to restart and then the splash screen reappears and the loop goes on.
What to do?
According to this site, I am eligible for email support on both products, but unsurprisingly, there's no email address anywhere. There's an option for a new support ticket but that just goes in a loop back to the support entry page.
I'm supposedly eligible for phone support but there's no phone option listed anywhere that I can see.
I know this is standard practice on many "support" sites and I did not expect much from Parallels but they have exceeded my low expectations.