Maybe I am wrong but it appears that for some reason my posting privileges are reset after successfully posting.
After logging in this afternoon I was immediately able to reply to 2 posts in 2 different threads with both threads created by other users. I then tried to reply to a post in another thread and got the "Unknown Error" message. Since then I made 3 attempts to post replies in various threads with no success.
It appears that maybe for some unknown reason my posting privileges are changed. I have no idea what might trigger that.