Warning when uploading post: GENERAL 500 … url=/checkDocument

For a while now I get a popup message whenever I edit a post and click on “Update”. Can anyone tell me what it means?

(I wonder if it might at all be related to an issue my webhoster has alerted me to: my website seems to create a huge number of tmp files in the hoster’s /tmp partition.)

My website is https://www.evamusby.co.uk
The message that pops up is:

GENERAL
500
At: https://evamusby.co.uk/wp-admin/admin-ajax.php?action=proxy_atd&_wponce=8f460bd262&url=/checkDocument

followed by an OK button. Once I press on OK everything seems to continue as normal.
Thanks ever so much for any help on this and please use idiot’s language as I am not good at this stuff.