The last few days, a bug in the system caused the captcha to appear all the time for everybody, sometimes even in an infinite loop.
This bug is fixed now, so things should be back to normal: the captcha may appear sometimes for some users, and should be working.
Still, occasionally it can happen that the captcha is causing trouble for some users. In that case, write a PM to oss
including your IP address and username (if different from your forum username).