In summary I needed to select at least one domain from each of my certifications or my CPE submission would just disappear without a trace or any notification something had not worked correctly.
I've just tested again and I can submit CPEs just fine as long as I select at least one domain from each of my certifications. If I don't do that, instead of my CPE submission disappearing, it now gives the error you're describing (see screenshot below).
The badge system on here says you hold the CSSLP, so our situations are different in that respect, but similar in that neither of us fits the more standard profile of someone who holds only the CISSP, so I'd wager that has something to do with it.
@Kaity - Hi Kaity - can you feed this back to the team investigating the CPE portal issues please?
I am no longer seeing this error message when submitting CPEs against only one of my certifications (which was the only time I used to see it), so this appears to be fixed now!