I remember that some weeks ago when captcha was activated for retyping the password we discussed the problem in the german IRC and I remember that there was at least one guy who could reproduce the problem. His way to solve the problem was just by trying over and over again. Which is actually a bad way. Anyway, the problem might not only be related to client or server but also to the combination of client and server (and possibly what is inbetween).
Could it be that there is a mistake in the captchas? The picture and text not being the same so when someone types in the captcha it tells them they got it wrong. Trying again and again would give you a different captcha each time until you eventually get one that is correct, and you can then log on. I don't know if different languages have different captchas, but if they do, that would explain why it appears to be a german problem only.