Dreamweaver CS6 and Dreamweaver CS5.5 use the Subversion 1.6.9 client library.
Dreamweaver CS5 uses the Subversion 1.6.6 client library. Later versions of the Subversion client library are not backward-compatible. If you update a third-party client application (for example, TortoiseSVN) to work with a later version of Subversion, the updated Subversion application updates local Subversion metadata. Dreamweaver can no longer communicate with Subversion.
You can, however, generally use multiple clients/servers of Subversion successfully as long as they're all the same major.minor version. For example, 1.4.x works with 1.4.x (and 1.4.5 works with 1.4.6), 1.6.x works with 1.6.x (and 1.6.6 works with 1.6.9), and so on. Errors can arise when you update files with a third-party tool that automatically updates your local Subversion meta data to a later version. Then, Dreamweaver becomes incompatible with Subversion.
Dreamweaver CS4 users can use a downgrader extension that restores metadata to the previous version of Subversion. For more information, see Error: "SVN: #155021. You cannot update this file using Dreamweaver's Subversion integration... " | Dreamweaver CS4.