Hi @nijo,
Try this:
certbot register --update-registration --email blabla@blablabla.tld
Where blabla@blablabla.tld is a valid email address different from your previous one. If you unregistered your email address, you can't use again the same email address, well, you can, but even if the process ends correctly, you won't receive mail alerts, so, add a new and different one.
If this process doesn't work, maybe @jsha or @cpu could take a look to know what is happening.
Cheers,
sahsanu