I have the same problem trying to answer some threads.
First time I tried to answer here I got it, so not related to age.
Quite annoying after taking the time to compose a message, I will start posting just “TEST” first to see If I can reply, and then edit into a proper reply if it works…
Since more than one month, I often use to get this “You don’t have permission” message.
It is very annoying.
Then I found a workaround.
After composing message, I copy-paste it in a text editor (notepad) and save it on my windows desktop.
I then click “Submit” button. If message is posted, I am lucky, and delete the saved text file.
If not, I just exit and after 15 minutes try again, copy-pasting from the saved notepad, and click Submit. If it still does not go through, I wait another half an hour and try again.