I hope this will be helpful rather than the umpteenth commenter with the same thing. I was getting the 403 error on Windows 10 (browser Firefox, with or without add-ons activated), and iOS 15.6.1 (Safari).
Okay, I tested, and tried to reply to a comment on one of my posts from today just now and it worked fine! (iOS)
ETA: This comment also went through on the first try. Hooray!
ETA2: Okay, so the comment went through. Then I went to edit it to tell you that and got the 403 message upon hitting 'update reply'. I needed to refresh the 403 page once and then the edit was posted. The second edit went through on the first try. How strange.
no subject
Okay, I tested, and tried to reply to a comment on one of my posts from today just now and it worked fine! (iOS)
ETA: This comment also went through on the first try. Hooray!
ETA2: Okay, so the comment went through. Then I went to edit it to tell you that and got the 403 message upon hitting 'update reply'. I needed to refresh the 403 page once and then the edit was posted. The second edit went through on the first try. How strange.