I’ve been using version 0.17 for a while and I just noticed the problem today. Looks like maybe OSM changed it’s upload URL and merkaartor (which doesn’t seem to be actively developed) hasn’t updated this.
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?
It seems that it was the combination of an old bug in Merkaartor and an configuration change on the OSM servers which disabled their workaround. In the recent hours they changed it back and my test upload a few minutes ago worked again. Now i can edit the rest of the data in the afternoon and upload a lot more.
I dont crave so much for the newest software versions, Merkaartor 0.17 is already good enough for me (except for multipolygons which are an problem of their own).