Update 3.2.0a1 to 3.2.0a2 --> Error: CANNOT_DELETE_FILES

Hi,

i have installed a phpBB 3.2.0-a1 and was going to update it with the update package: https://download.phpbb.com/pub/release/3.2/unstable/3.2.0-a2/phpBB-3.2.0-a1_to_3.2.0-a2.zip

I put all those three folders to the root of phpBB3.2.0-a1 and browse to: phpbb32/install/app.php/update

At step: "!Update files" i got the error: "CANNOT_DELETE_FILES" in a red status bar. Below is only one line: "Skip “installer.update_filesystem.diff_files” task"