Odd, I know its very unhelpful to hear that ‘it works for me’ reply, but … it does, and on what appears to be a very similar system.
Anyways, I hope this helps. ie, be methodical and do a virgin install.
Earlier versions of Nuendo may be downloaded here.
Re. the re-booting and hardware peculiarities of the 7.1MP: Odd machine at times, in any case, if and when this happens, do reset the SMC by pulling the power cable out for a few minutes, then back in and re-booting.
Uninstall and in particular, delete all of the Nuendo preferences folder. Reboot, clean install & when launching, let it re-build all of the Nuendo preferences again. While you’re at it, disable all third party plugs before 1st launch by re-named the relevant audio plugs folders to ‘unused’ or similar. The latter has not really proven to be that much of an issue for me, but worth trying in a clean install. What I will say is that clearing out Nuendo’s prefs has usually worked wonders with any glitches in a new version. This especially includes N10.3 - I do keep the project templates then relocate them to the prefs folder without issue.
Where this has seemed to work best is when older projects can have bugs when loaded. With clean prefs all of my projects seem fine & this includes those with (say) large VI counts (NI, Absolute 4 etc), film soundtracks with video timelines etc. Only one here now that remains with some odd bugs but it would seem that is directly related to the extensive use of (buggy) LoopMash in that particular project.
Otherwise, I would say that Catalina has been a real dog (many issues to report about there), however, after two botched updates - 10.15.4, then [ahem] a ‘supplemental’, then 10.15.5 and yet another supplemental … this one seems much better than earlier, or at least more like what we should have got in the first place. Personally, I would have much preferred to stay with Mojave, but of course Apple will not let us do that on new machines …BTW on my dual Xeon Dell Workstation /Win10 for Workstations 2004 /RME UFX+ - none of this applies and Nuendo works perfectly (albeit perhaps a fraction slower /greater overheads).