I've tried two desktops, Macbook Air (Safari and Firefox) PC (Firefox, and Pale Moon). All are the same.
It it helps I have found a work around. The problem seems to always relate to quoted posts, delete the quote in the text and the problem seems to go away.
Using quotes pasted into the "Post Quick Reply", "Go Advanced" or the yellow "Quick Reply" button at the lower left of the page seems to cause the problem from time to time, whereas using the "Quick Reply" button to the right of the "Quote" and "Multi Quote" buttons hasn't yet ever caused this to happen for me.
The key seems to be quotes, from what I can see, though. I'm afraid I'm not very computer literate, so can't help much further.