Joomla 1.5 user registration problem needs solving

For some reason the registration and account approval procedure on the Joomla 1.5 website of our customer doesn't work as it should and we cannot find the solution. Those who register normally get a confirmation email and after clicking the link in this confirmation email, they should be able to log in. Not so any longer. They still get the confirmation email but clicking does not automatically activate the account. Even manually setting the account to 'activate' does not actually activate the account. We have tried to manually add an account and this does not work. The trouble started when we had a 'captcha' inserted to avoid robots registering. This is a very simple job for a knowlegdeable freelancer. Preference for a DUTCH speaking freelancer.