My site(3.3) worked well and I am happy with its performance. Unfortunately, the trouble came after I upgraded it from v.3.3 to v.3.4 two days ago. Only one account logged into it, but he got an error message "ERR_EMPTY_MESSAGE." Pressing "UNLOAD" button can't solve it. I ever tried to flush DNS. At the beginning, it worked fine, but it failed again after a while. Any suggestion?
Moodle version: 3.4+
PHP Version: 7.1