Sorry, with the next release, the old domain sandbox-cryptpad.domain.tld is no more used. But the correct « domain » for sandbox is sandbox.cryptpad.domain.tld.
There is no domain sandbox.cryptpad.domain.tld created, but a a CNAME must be added. And the certificate must be renew, but perhaps it needs wait a bit to have dns cache and propagation renewed before. I have get similar error trying renew the certificate, but after a while, I could renew it…
Also you can remove the old the domain sandbox-cryptpad.domain.tld no more necessary.