You can try a few things:
* use the repo browser dialog and check if there are files with weird names in the folder which is your working copy root. If there are files with chars in their names like "/\?*" or filenames that start with "con", "prn", "com", then that might be a hint. Delete those files directly in the repo browser.
* try to check out a fresh working copy
* configure your virus scanner to leave the TortoiseProc.exe process alone, and also your working copy
Usually, this error happens due to interference from virus scanners.
If that doesn't work, you should ask on the svn users list for help, since the error is from the svn library code:
http://subversion.apache.org/mailing-lists.htmlStefan