Hehe, well, it's getting funneĆ½ here..
I have done nothing but have the lappie sleep overnight, and things have changed; see below.
I must conclude that even a reboot wasn't enough; upgrading .NET from 4.5 to 4.5.2 required the lappie to sleep on it
Anyways, I can now select any Device with no errors.
However, only the MP-1 and Classic presents a full window of parameters; selecting the other ones results in a small'ish rectangle with no params.
Having selected either MP-1 or Classic and randomly changing MIDI channel, Program number, Loop, some randomly selected parameters.., I can save a .sysx file.
Saving .sysx on other device results in the same "Cannot save to file" as stated in previous post - but no further error messages or exceptions.
I can also [re]load the saved [MP-1 or Classic] .sysx files.
(EDITing done for adding clarity from test to test)