Due to the upgrade of the subversion server last week, some users may experience problems with merge/branch actions. These problems arise when a user only has access to a subdirectory of the repository. See http://subversion.tigris.org/issues/show_bug.cgi?id=3367 for more information.
A workaround is to give the user read access to the root of the repository. You can limit the intrusion to your repository, by using a snippet in your access file similar to the one below:
## BEGIN svn 1.5 workaround [/] * = r [/projects] * = [/tags] * = [/branches] * = ## END svn 1.5 workaround