Hi Martin,
When I first tried flats with the 2600mc (with dark-flats) I found my flats were badly over-correcting.
The solution (which I found on the internet:
cloudynights.com/topic/731949-zwo-asi2600mc-pro-flats-cause-issues/page-2) was to use flats & bias frames & this seems to work well for me (so far at least).
I don't bother with darks.