I did resolve the problem, also due to your most welcome hints in Studio 2019 repair sent to be earlier. Similarly, I now entered c:\ProgramData\Package Cache\SDL\SDLMultiTermDesktop2019\modules\.
Next, I clicked the relevant .msi files and this worked, reinstalling MultiTerm 2019 now without popping up error message as experienced. Then a program update took place to render the program to its latest version. This settled the whole issue with the popping up error message.
Take care and have a nice weekend, Hans-Georg