I checked our logs and as far as I can tell, it’s caused by an issue at Let’s Encrypt. The error message is quite weird and I can’t think of another explanation.
I see the exact same error on my account. Renewing my existing SSL certificates do not work. I have 7 days until my SSL certificates expire. Will try again tomorrow but if you want to, do have a look at logs for my account as well.
There seem to be major issues with let’s encrypt ATM according to https://letsencrypt.status.io/. So the problem is definitely there and not at Infinityfree.