If you claim a server you must solve a captcha.
These captchas can be bad to read, and it happened that the page does not know what the captcha shows.

For me there is no sense in a captcha for claiming a server, since you must be registered at gametracker which should be enough captcha.

The CAPTCHA is simply an extra layer of security. It would serve the same purpose as a registration CAPTCHA - to prevent spam. Albeit minor, it does make your server page a bit more safe from spammers.

Now, if you're having trouble getting it to accept the correct text, GT would need a screenshot of the CAPTCHA image as well as the text you are entering. When you next need to claim a server, I recommend taking a screenshot of the image and your entry prior to clicking the button. Should you run into issues, post a bug report with the screenshots for us to determine if it is a bug or not.

Actually the problem doesn't show, might be that it happens when you wait for the server to be claimed, but hit the "claim" button again after waiting time.