I have the same problem. I bought a domain already parked there. Since “an error occured” I changed to PC and wrote a mail before changing My nameservers.
They replied after 1 day that if I wanted support I should change the NS to afternic (see the language here). Immediately I changed NS and after about 8 hrs got page displayed as available for sale at afternic ( i put up for sale as well) but no ads.
Waiting eagerly for ads to show up, it went back to “an error occured” and I wrote mail again.
They replied after some 16 hours that due to GDPR they are unable to verify ownership.
I will send them screenshot from registrarto show my ownership.
I hope it will take another day or two for them to verify.
I thought about trying GD cashparking meanwhile. Not sure what to do though.
The domain earned decent revenue before I purchased, it was making more than $1 per click and about $4-6 a day.