Ubuntu one. Windows client in Windows 8 prevents many apps from copying in sync'ed folders

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http://askubuntu.com – I have Windows 8 machine with Ubuntu One client (seems to be 4.0.0, released on 2012/12/11). I have checked that when synchronization is active I can not write any file in the folders sync'd. Curiously, I CAN copy files in the folders using file explorer but I can not, for example, download a file there using web explorer, chrome, firefox, acrobat reader, etc. Any app that opens a "Save as..." dialog, completes the task without error messages but the file does not appear in the folder. If I disconnect synchronization, everything works well again Any clue? thanks in advance (HowTos)