It's not due to an incorrect certificate! If that were the case, the TLS connections wouldn't have made it further than the TLS handshake. But as my Wireshark network dump clearly shows, the handshake is good, the server just resets the connection a certain amount of bytes in the upload process.
I have same issue after latest CWP panel updates. I live same problem all web sites using Let's encrypt free SLL on CWP panel. Other SSL certs not have this problem. I'm contact with my other friends using CWP and Let's Encrypt SSL. All is have same issue after latest CWP updates.
Maybe this is not Let's Encrypt SLL issue but now we live this problem with CWP panel + Let's Encrypt SSL
I try contact with CWP technical support but I must pay credit for get technical support. Maybe you are too try contact with CWP technical support about this issue.
Problem still exist. Now I dedect using any SSL certs websites with latest CWP updates live same problem. Problem not cause of Let's Encrypt SSL. All SSL cert not work correct after latest CWP panel updates.
I thing this is latest CWP update BUG. POST data with SSL not work cause of this BUG. I thing any incorrect limit set after latest CWP update. But I can't find which limit set wrong with latest CWP update automalicly.