Question: I'm trying to `svn update` my SVN working copy with TortoiseSVN but the update fails, asking to perform the clean up first.

However, the `svn cleanup` fails too stating that a `\.svn\pristine\20\20long-namecbf.svn-base file cannot be found`. 

I've already tried to release the lock but this does not work either.

**Any idea how should I proceed?**

Answer: 
I got away by replacing the .svn directory with the same from a freshly checked-out copy. Fortunately for me, the local copy was on the same revision as the repo. Not sure what the implications would be if it was not the case.
For me the file was there but had (1) appended to the name. (A copy)
ff4c7be3da460bb91144d6761cc5743536c983f4 (1).svn-base

I removed the "(1)" and cleanup ran fine
ff4c7be3da460bb91144d6761cc5743536c983f4.svn-base