Problem uploading with Merkaartor
If you receive errors with number 401 (bad authentification) when uploading new data to Openstreetmap with Merkaartor, you may be running into a known problem with Qt. More specifically this bug is only fixed in version 4.3.3 of Qt (or more recent).
If you are on (K)ubuntu, you can download pre-compiled Qt 4.3.4 libraries. Unpack the file and then execute the following commands
cd qt4-434lib export LD_LIBRARY_PATH=$PWD
Now you can start Merkaartor again. It will pick up the newer libraries and (hopefully) have no trouble uploading to OSM.
Self compiled newer Qt
On other systems you can easily compile your own Qt. Download the latest source version (at least 4.3.4), unpack the file and execute the following instructions inside (Note that you need to have write access to /some/random/location; for example make it a subdirectory of your home directory)
./configure -prefix /some/random/location make make install cd /some/random/location export LD_LIBRARY_PATH=$PWD
Now you can start merkaartor and it will use the newly compiled Qt.