I received a support call because a repository named xxx_y+_z was inaccessible.
There was no error log, only information indicating insufficient access rights or that the repository was unavailable.
I copied the repository to the file system under the name xxx_y-plus_z, rescanned it, and lo and behold, it worked.
So, it seems to me that the plus sign in the repository name is the problem.
It is possible to checkout this repo via svn, but not browsable in rhodeocde