Hi again,
while I am still interested in whether Cryptomator would directly work on ARM with above mentioned OpenJFX I meanwhile found the solution to my problem 
The simple answer is: just use the Cryptomator CLI …
While it is labelled “early stage and not ready for production use” a few first tests worked just fine on my armbian machine.
One thing I would like to know nevertheless: currently version 1.5.0 is underway and I found some remarks about incompatibilities with previous versions (new vault format), so what about the CLI version when 1.5 becomes released?
Will it still be compatible? (I doubt that)
Will there be an adapted CLI version available the same time? (that would be really great! 
)
Or will I have to stick to the old version when I want to use some ARM based machine for that purpose?
Any information about that will be greatly appreciated 
Kind regards
Harry