The response is not a valid JSON response.

I had this problem and it took some time, but eventually, I figured out what was causing it (in my case). As a premise, I did literally everything to fix this, from updating (everything plugins, php, WordPress, etc), disabling plugins, changing themes, resetting permalinks, etc. with no results. I contacted server support, and their response was super simple: nothing wrong on their side, but if I pay a premium support service, they can fix it. At some point, I’ve been hit by malware, I contact server support asking about what kind of security they provide, and they gave me the same answer. I decided to run a scan. The php.ini in the root, in the folders wp-admin and wp-includes, have been changed. I fix them manually and I resolved the problem. I couldn’t find any documentation about the relationship between malware and The response is not a valid JSON response.

This is what I found:
safe_mode = Off
disable_functions = NONE
safe_mode_gid = OFF
open_basedir = OFF
exec = ON
shell_exec?=?ON

This is how I changed it:
safe_mode = Off
disable_functions = system, shell_exec, exec
safe_mode_gid = Off
open_basedir =

It worked for me and I hope it will work for others.