Good news
I found some explanation on http://lists.openstreetmap.org/pipermail/merkaartor/2012-May/date.html
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).
Happy mapping!