Hallo Jan, Dutch here too.
Coming from Aldfaer. Lots of errors at first also. But when you look closely it might be _xxxTAGs. So tags from Legacy itself, non standard Gedcom tags. They start with underscore.
It is what Zurga said. if there are patterns, they are more then willing to adapt the import for Legacy.
In the near future there will be a complete dutch guide too (only partly now), working on that. Hopefully more about importing and conversion also.
Getting your Gedcom to convert is sometimes troublesome, because lots of programs seem to use their own "Tags", not complying to the Gedcom standard.
But Zurga does a great job in getting the most out of it, so stay tuned!
There is a lot in the pipeline.
What version are you using?
Tineke.