Switch back to ssl.com
Apparently zerossl.com does have limits on its free account:
Note: Your account is holding more than 100 ACME certificates. For using the Dashboard or API with this amount of ACME certificates you have to upgrade at least to the ZeroSSL basic plan.
This is not surprising by itself, but it was not advertised on their website. We'll try switching back to ssl.com for now, because allegedly their ACME problems have been fixed by now. Getting a paid account would also make sense for our CI needs I think.