Thanks for the suggestions above. Just to make sure I tried the following:
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I cut the large external display completely out of the system (removed DP cable) so that I had only 1 external display with the same resolution as the laptop (i.e. 1920 x1080) and still SU Pro freezes when started up on the external display.
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I tried other cables as well with no effect.
My conclusion is that we might be going up the wrong alley here. The laptop works perfectly with the two external displays for all other applications that I use. There must be some specific GPU/Laptop/SU clash that causes SU to freeze. Maybe I should start a new technical problem topic to try and resolve this technically?