Subversion Merge "Path Not Found"

In my Subversion project I have branched from the trunk, made some changes and I am now reintegrating that branch back onto the trunk. When I try to do the merge, I get the following error message, which appears to be complaining about a particular revision.

Merge status operation failed.
Filesystem has no item
svn: '/svn/repo/!svn/rvr/24066/path/to/branch/newProject' path not found

Note: The branch was created at revision 24068.

Can anyone explain why this error occurs and how it can be resolved?

EDIT

The version of SVN and Subclipse I am using is 1.6.

Solution

For anyone who happens to stumble upon this question, here is a few things that I have picked up since posting this question:

Check the path it is saying doesn't exist actually exists, svn is usually right about these things.
It can sometimes be a case sensitivity issue, so check for those rogue capitalisation mistakes.
If you are using svn over http, paste the url into your browser and check that you can see the file.
Try doing a svn cleanup or in eclipse Team > Cleanup. This is designed for removing locks which are left from when an operation doesn't complete successfully, it has fixed a few head scratchers for me in the past!
Check for rogue svn.info files. This file should exist in the root of your branch, and can sometimes cause problems if it exists in other areas of the project and isn't kept up to date. I saw this problem at work on 26/06/15 with svn 1.8 and eclipse with subversive and the svnkit 1.8 connector, which was preventing us from merging. The solution was to delete all the svn.info files except the one in the root of the project.