In the meantime I was able to solve the problem and installed Cryptomator 1.14.15.
To find out what was preventing the installation, I stopped all running programms from Startup or otherwise running (PortableApps …) - to no avail. Then I removed all plugged in USB devices as printer, memory sticks, external USB-HDDs. Since I remove those always over «save remove button» down in the task bar, from one external HDD I got the message it can not be removed, it’s in use by some (unnamed) program.
So I powered down my PC, unplugged the USB-HDD and repowered.
After that I was able to install Cryptomator as it should be and reactivated even the encrypted Cryptomator-folder from my old NB inside the Dropbox folder located also on the same external USB-HDD.
Obviously this USB-HDD (or the Cryptomator folder on it) was the source for the trouble.
This is somehow strange, since I could access the data stored on this HDD (lots of pictures) before and after the installation with no errors at all.