I have the same issue since some days on Debian Testing (Wheezy) with Merkaartor 0.17.

However, everything still works fine on Debian Stable (Squeeze) with Merkaartor 0.16.

The Debian package for Merkaartor has not changed in the last days so the issue must be somewhere else (ie either on the server side or in a dependency of Merkaartor)
@Sat: which distribution are you using?