Just to understand it, does Zorro uses timestamps of the historic data always as UTC? Could i work with offsets in this case? (1 or 2h depending on dst) to get the "real" UTC time from my data? Or is it best to convert it?

The problem seems to be be that the data back in old history is shifted even more, so somewhere between there was a change.

How long is a trade week? Always 5*24h (in forex) or did it change over the year(s)? If so it would be possible to find the shift (by looking at first and last bar of a week)