I have had the same issue and have been pulling my hair out for the last 5 hours trying to fix it. The forum here was no help… so I did a Google search and found the answer… for me at least. The problem was in the permalink setup. Try the following…
Go to Admin – Settings – Permalinks. If you have set up a Custom Structure… put the cursor at the end (right side) and make sure there are no trailing spaces. (i.e. delete any spaces after the end of the custom structure). Save the settings and try posting a comment again.
Hope this helps.
