> I started Grunch which told me: rexxsyslib was not installed.

This Grunch message *may* mean that there's no *working* rexxsyslib.library (i.e. m68k one) installed. But I'm not sure about that.

> then I installed it.

...by which everything should have been fine using Grunch. There was no need to manually install yet another rexxsyslib.library from anywhere, then.

> In MOSSYS --> Libs, there was and is still the dummy. This is correct
> or should there be also the proper rexxsys.library?

As is documented in various places in the MorphOS Library, if everything is done correctly (either manually or via Grunch), you end up with an m68k rexxsyslib.library in SYS:Libs and a MorphOS-native rexxsyslib.library_disabled (or similar) in MOSSYS:Libs.

https://library.morph.zone/Tips_and_Tricks#Modifying_the_System_Directory

> what actually almost scared me was the fact that so many applications
> did not work any longer only caused by an old version of rexxsys.library??!!

As far as I understand, this wasn't because you installed "an old version" (it's no older than the others), but because it was PowerPacker-compressed and thus caused trouble being loaded in non-uncompressed state by programs using rexxsyslib.library.