I made progress in hunting down the annoying noise blob (see other images for examples). If i turn on the cooler of the ASI294MC Pro the noise blob will appear after a few minutes (images C and D). If i turn the cooler off, the noise blob disappears after a few minutes (images A and B). Just compare B and D and you will clearly see the noise blob. All images were taken with a flat field box and under exactly the same circumstances - just with the difference of the cooler.

Image E shows the result: This is NGC 7023 after about 90 minutes of total exposure. No calibration frames were used, i only applied background extraction and a stretch. It is easy to see the blob :-(

Version A: The uncooled flat (stretched). Everything fine.

Version B: The uncooled flat (stretched and background extracted). Everything fine.

Version C: The cooled flat (stretched). Annoying noise blob visible.

Version D: The cooled flat (streteched and background extracted). Blob clearly visible.

Version F: Example image with the noise blob :-(

I think it is time to contact ZWO about this issue. I will keep you informed.

[Edit]: Link to the thread @ZWO-Forum:

ZWO Forum

[Edit2]: Katherine from ZWO told me to redry the chamber. I did this and got rid of some dust, but the main problem remains. It is not a problem of frost or humidity :-(

Version G: Cooled, after the redry-process.

Version H: Cooled and background extracted after the redry process.

[Edit 2019-07-27]: With further help of Katherine from ZWO we were able to track down the source of the blob. Like pmumbower suggested it is a freezing problem, but the humidity is on the CMOS-chip itself - not on the protection window before the chamber nor on the protection window directly before the chip. Directly on the chip :-( I just sent the camera to ZWO via my local dealer TS, because i can not repair the problem, it needs to be fixed by the manufacturer itself. So, everyone - keep your fingers crossed that they can get rid of it and that it won't take a lot of time (shipment from my place to TS (germany), from TS germany to china, repairprocess itself, shipment back to germany...)...

I attached another image (I) of the problem while the cooler is running at 100 %, but it is nearly impossible to take a photo of a CMOS-chip due to the reflections. With the naked eye the blob was clearly visible on the chip.

So the good news: We found the source of the blob!

And the bad news: I won't image for a while due to the lack of an imaging camera :-(

[EDIT] UPDATE 2019-10-01

I am back in business! I just got the camera back from ZWO and it looks like it is working (see image J). They had to change the whole sensor, so i guess something was really broken.

[EDIT] UPDATE 2020-10-06

The blob is back. See more information here