I migrated some new files from trunk to a branch by copying directly from one project to another (drag&drop). New files in branch project aren't recognized as a part of SVN and can't be committed to the repository.
This happens only for my non-Java files, they're copied fine, but JSPs, JSs, GIFs etc all have a problem.
Ok of course this was a pain for a while now, but the answer occurred just as I posted this.
What Eclipse doesn't tell you is when you copy folders between projects, hidden folders are copied as well. This includes .svn folders which contain svn data. Once I got rid of those in my newly created folders, my problems were gone. I just have to find where to ignore .svn folders when copying files and folders in Eclipse.