Cubase 9.5.2 Pro - Freezes on mixdown

Windows 10. Freezes on start of offline mixdown at 0% and button is blank (‘cancel’ I think). Got it to work once only.
32 GB of RAM and never goes above 50%. Tried all manner of resolution and depth.
Got a CPU overload message when trying to mix in real time. Core i7
Rebooted PC - no avail.
Cubase 9.5.21 and same for 9.5.10 beforehand.
Steinberg UR44 and that seems to be working correctly.
I was trying to submit a ticket to Steinberg but went through a roundabout of websites and got nowhere. How can I submit a ticket directly?