Hi!

I'd say you are backuping a source file that has a very long absolute path, just under the limit, to a destination where it will have an absolute path above the limit.

To elaborate. NTFS is limited to an absolute path of 260 characters (with a few exceptions.)
An absolute path is the complete path including everything. For example, c:\windows\mytempfile.txt is an absolute path with the lenght of 26 characters, everything from c to t is counted.

So, say that you have a source file with an absolut path of 255 characters. Thats just fine but when cobian backups it to, for example, d:\backup\ you add the lenght of backup\ (7 characters) and suddenly the file has a absolute path with the lenght of 262 and windows will refuse the request to create the file.

This is a common limitation of backup softwares that use a pure file copy. To get around it you have four choices
1. Switch to another backup software without the limitation (not recommended )
2. Use compressed archives which will have their own internal folder structure which does not take the compressed archives absolute path into consideration. There are a few drawbacks with this technique so i prefer not to use it myself.
3. Try to make the paths to each backups root folder as short as possible. CB will still add quite a few characters when using timestamped folders and such but you might be able to make it work.
4. For gods sake man! My longest path is 112 characters. How in the name of f**k did you come even close to 260 characters!? Clean up! :-P

You can read more about path lenght limitations here: http://msdn.microsoft.com/en-us/library/aa365247(VS.85).aspx#maxpath

Cheers