Gents, I do appreciate your support, really!
Well, I just checked first the folders that you, hartpaul, indicated. No, nothing like that. Well, the folders exist but the content is only the basic type. Thanks anyways.
Then levifiction, I convinced windows to log as described in your suggested article, just ran some random installation and got the first MSI***.log. Now I will dive in deeper, select the right settings for installer logger and then start analyzing the logs. (BTW: I ran the installation for the Mega Script bundle with a) no PSP application installed which I flushed EOW to start over freshly, b) almost all logging flags set and so the logger spit out a log of almost 4MB. Guess I need to find the important settings first

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Here the state of UAC: No, I did not disable UAC but I set it to lowest. I might also try completely disabling UAC manually for the time of the installations.
And yes, that Purple Tint I downloaded as well and it installed correctly.
And guess what: I got also an empty folder Languages on my drive C: even if the packages' names sub-folders have not been created.
I'll check for errors in the log files once a representative has been created.
Getting closer, hopefully?!?! Good hints in any case!! I'm so curious....
Ok, That will keep me busy for a while. I'll let you cracks know how that went.
Danny
Edit more than a year later: Couldn't fix it and didn't find more than was installed by clicking around. Than I ran the "retrieve my orders". That did actually a lot more even though I still didn't fin all I thought I bought. But the ... see my post end of '21.