I get this error message from SVN:
svn: Attempt to add tree conflict that already exists
Now, this has already been asked on this site and the given explanation is that a tree conflict has not been resolved after a previous merge, and now SVN is trying to add the same tree conflict again.
So I called "svn status" and marked all tree conflicts as resolved. I then did svn revert -R * and double-checked that the output of "svn status" is completely empty. I then tried the merge again, with the exact same error message at the exact same place.
It seems to me like svn tries to add a tree conflict in the very same place twice during the same merge operation, which, with all due respect, would be a severe bug in SVN.