POST forms return certificate error with Let's Encrypt
I've recently set up an SSL certificate with Let's Encrypt on my Droplet. It's for three domains in total (two
www domains and a subdomain for one of them,
files). I'm using PHP7 and Apache 2.4.17 from a PPA, and I've also set up HTTP/2. The certificate was created on 19 March and it expires on the 17 June, though yesterday some problems popped up. Whenever I submit a form both Firefox and Chrome most of the time report an error with the certificate (Firefox is not at all specific, Chrome mentions an
ERR_SPDY_PROTOCOL_ERROR). I first noticed it on the login form, and it only seems to happen when I enter at least a valid username. Just before I noticed the problem for the first time, I switched my users table to use UUIDs instead of auto incrementing IDs. Could that, or maybe setting cookies, be causing the problem? I doubt it because it worked before.
I'm really not sure what to do here, should I delete the certificates an generate new ones?