The response is not a valid JSON response

On my site, I can’t upload images to post and pages and save the changes. WordPress showing the following error:

The response is not a valid JSON response

So far we’ve done:
– Changed and re-caved Permalinks
– Cleared server cache
– Increased Max_execution_time
– Change Themes
– Changed PHP version
– Tried in a different browser and in different devices

Please advise.