Dear Pawel,
Could you try to install the command line tools in TortoiseSVN and then try
to cleanup in the command line?
Something like this:
cd /d [drive and path to your repository]
svn cleanup
If this works then you should report the error on the mailinglist of
TortoiseSVN,
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Subversion Exception!
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Subversion encountered a serious problem.
Please take the time to report this on the Subversion mailing list
with as much information as possible about what
you were trying to do.
But please first search the mailing list
