I'm getting the same error as in this thread https://www.sidefx.com/forum/topic/69036/. [www.sidefx.com]
The solution given is down to the fact that H18.5 used Python 2.7 - but H19.5 is now Python 3.9.
However, when I uninstall Arnold it works fine.
Presumably It's trying to use Arnold CPU as opposed to Mantra?
For now I'm going to make do with 2 different installs but I'd rather not!