Sophos Migration Assistant will not import any backup from UTM

So are you migrating from UTM to XG? If not, then what are you using the migration tool for?

Yes. Trying to move to XGS Appliance.

Two ways I would suggest this.

One would be to get a Sophos Partner involved in the migration, as they have access to the partner migration tool.

Second, I would honestly not really bother with the migration tool and just build it from scratch.   There is so much difference between the two products, you will spend more time messing with the tool than you would rebuilding your XG.

Well, great time for home users i guess.

What is your goal? 
Because you can do everything even better with scripts and other tools, if really needed.

But the initial installation of SFOS and the first steps are not that complicated to begin with to get you up and running. Then you can build your rule set as you go. 

You could also check out this:  Sophos Firewall: PowerShell Script Library for migrating Sophos UTM to Sophos Firewall 

Mine really wasn't that bad to do, and once I asked a couple of questions on the XG forum, I have a decent setup.  Can it be tweaked?  Sure, but it's nothing breaking my setup.  It took a little more time to do some things, but overall, it runs okay for now, and adding Pi-hole recently to my network on Proxmox is taking care of ad-blocking.

Mine really wasn't that bad to do, and once I asked a couple of questions on the XG forum, I have a decent setup.  Can it be tweaked?  Sure, but it's nothing breaking my setup.  It took a little more time to do some things, but overall, it runs okay for now, and adding Pi-hole recently to my network on Proxmox is taking care of ad-blocking.

Thanks for answer to all of you but...

There is an tool available that doesn't seem to work for me and I ask for help for that tool. Either Sophos offers a tool that works and give support if not or better not release it. I am familiar with SFOS and I will be able to decide if the result from that tool is the way to migrate or I will simply start from scratch (what also helps to reduce orphained objects) or use Powershell script library again (I formerly used).

So I ask again: I cannot start with that tool because my UTM file is not accepted. Any ideas to break through that wall or not?

Best regards Gernot

Which is why I suggested that you contact a Sophos Partner, as this is a community forum that we do not have access to a migration tool.  If you would read the site for the migration tool, Firewall migration - Migration center (sophos.com) that's about all it will give you - a Sophos Partner, or a migration helpdesk link.

The only thing we have available to us as non-partnered users is the Powershell Script Library that was linked above.