Dear Martin
thanks so much for offering your advice.
I have found a method to replicate the problem:
I have two different accounts on my WIn10 system. Account A is a DAW specific set-up with most services you typically find in autostart being disabled (dropbox, adobe updater etc.). Account B is an MS-Office-focussed setup.
I only observe this behaviour when I swich from Account B to Account A. If I then shut down Cubase and restart it the menues are displayed correctly again. Seems like a bearable work-around.
The bug only started to occur after I installed VST Connect. This is most likely a mere coincidence but I thought I flag it up anyway.
My graphic card is a pretty standart NVIDA GT1030