I have somehow managed to change the remote URL of the SVN repository. Thus, Idea/svn now tries to commit my project to the root of the repository instead of the subfolder that I actually have write access to. How can I change this back? (The URL was originally correct and managed to commit correctly before)
Repository URL is stored in the working copy metadata, IDEA doesn't change it automatically. You can change it manually from IntelliJ IDEA using the VCS | Subversion | Relocate menu.
It can also be done from the command line with svn relocate or Tortoise SVN Relocate... menu.