That is to be expected until BigMuscle makes the release version, your donate key needs to be kept but does not register the experimental version. You will also see the watermark. Just hit cancel on the first notification, keep the DOS window running (it is harmless, just minimize it), and hit okay I believe on the next dialog. Correct me if wrong.
Hang tight, we are all waiting for the release version for 1903.
Thanks,
Netguru