Hello,
I was searching for older fimware-versions and could only found the newest (v556) on the victron portal as follows:
Any hint where e.g. version v552 can be found for download ?
There is a txt file with a link to an archive.
aaah, yes I found it
thank you very much for the fast reponse ! 
[Moderator edit: Victron Professional is the place to find the archive]