SVN post-commit error: cannot remove transaction folders

SVN post-commit error: cannot remove transaction folders

my problem is the following.
I have a network connecting several pc win both WIN (and tortoise svn)
Ubuntu and Mac (the last two using svn on terminal). Every time one of the
users tries to commit something, after entering the password (and after
the commit) we obtain this error:
Transmitting file data . Committed revision 3758. Warning: post commit FS
processing had error: Can't remove directory
'/nfs/svn/Adaptica/db/transactions/3757-1.txn': Permission denied.
(the numbers of course change every time!)
The commit worked so that if anyone try to checkout, he will find the
correct and updated folders and files. However, on the server (there is
Ubuntu 12.10 and the entire structure of svn has been copied from an older
working server to this new one), the folder transactions is full of
####-1.txn empty folders. These folders can be deleted only using sudo rm.
I can add that each of this ####-1.txn folder has permissions drwxrwsrwx+
and that the owner is the user that executed the commit (whit rwx access
permission to the folder), and that he is also part of a group with rwx
permissions to the same folder. Also, there are no hook activated.
How can I solve? Thanks!