Running :Rtags will fail on a new rails project that has no tmp folder in its root directory.
At least my new Rails 3.2.3 project did not create one, not sure if and when it will be created.
Manually creating the tmp folder solved the problem.
Can you check and create the folder before running ctags?
:Rtags creates tmp/ if missing
Weird, for every other empty directory it creates a .gitkeep file. At least it's easy to work around.