Can't edit post

Sometimes I try to edit a post and when I click Edit Post, I get the edit window but it is blank, instead of containing the text of the post. Sometimes this happens when I try to edit a post as a Moderator and sometimes this happens when I try to edit my own post.

It is intermittent and I cannot determine conditions that reproduce it. But when it happens on a particular post, it continues to happen for several tries in a row.

Before Edit.png
After Edit.png

I had the same a couple of times yesterday.