File Manager downloads failing (zip problem)

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Hi all,

I am new to using C5 and so far really liking it. But I am having no luck at all getting the File Manager to download either single or multiple files. Each time I get a 1kb zip file that does not open, even for single files.

My server is Win 2008 running Apache 2.2 and PHP 5.3.6.
[Edit: I have updated to Apache 2.4 and PHP 5.5.5 and the problem still exists]

I understand the Windows version of PHP 5.2+ has support for the ZIP extension built in and does not need the mod_zip extension enabled. Even so I have tried with and without this extension and the problem is the same.

I have also tried some other suggestions found on this forum such as adding:
define('DIR_FILES_BIN_ZIP', 'C:\Program~1\7-Zip\7z.exe');
define('DIR_FILES_BIN_UNZIP', 'C:\Program~1\7-Zip\7z.exe');

still I get the same problem.

There are 2 lines logged to the Apache error_log each time:
PHP Warning: filesize(): stat failed for C:/Apache2.2/htdocs/Concrete/files/tmp/EvgwlmWBw9iR.zip in C:\\Apache2.2\\htdocs\\Concrete\\concrete\\core\\helpers\\file.php on line 117, referer: <my domain>/Concrete/index.php/dashboard/files/search/
PHP Warning: fopen(C:/Apache2.2/htdocs/Concrete/files/tmp/EvgwlmWBw9iR.zip): failed to open stream: No such file or directory in C:\\Apache2.2\\htdocs\\Concrete\\concrete\\core\\helpers\\file.php on line 136, referer: <my domain>/Concrete/index.php/dashboard/files/search/

Is there anything I can do to resolve this?