I got that very error with TortoiseSVN under Windows. Apparently, the .svn folder has a small SQLite database file, which, if you don’t have write access to, will cause that error.
As I was using TortoiseSVN through the windows shell (the right-click) menu, to work around it I ran the windows explorer I used to navigate to the folder as administrator and everything worked fine.
Another solution, of course, would be to give write rights to the user you are trying to use TortoiseSVN with.