I guess I have found a solution (at least a workaround).
As was mentioned by MadMusicologist the problem seems to be the digiasio.dll file.
I followed his hints but it didn’t help.
What I did:
I renamed the digiasio.dll in the System32-folder to digiasio.dll.orig (just to have it available in case anything goes wrong) but let the one in SysWOW64 untouched (this is the version with the 2015 datestamp).
Now wavelab does not crash anymore when I close it and ProTools 2018.10 is also working .
Hope this helps.
Thankx for following up.
I shall return here any report how this effects Cakewalk. (Presently I am in another score and do not wish to interrupt that work, sorry.)
Ok this seems to help for Cakewalk. ProTools still rocking. Thankx again, everything fine now - until the next ProTools update 
But then I’ll know what to do.
And Acustica STILL has no clue. I guess I’m not surprised. Thanks for the workaround!