I know there have been multiple posts regarding this, but after scouring through the posts, I would like my particular situation to be looked at as it might throw some new light on the issue. I'm a developer for a company that hosts a number of sites on shared servers. For this particular site it is the only one on the server using c5, but the sites on other servers (which should have the exact same configuration as this one) do not reproduce the problem.

The error is:
An unknown error occurred while uploading the file. Please check that file uploads are enabled, and that your file does not exceed the size of the post_max_size or upload_max_filesize variables.

Uploading single files works ok, adding multiple files remotely works ok, it's just adding multiple local files that triggers the error. Also (this is the weird thing), everything works fine on IE8 but all of the other browsers trigger the error (why would such an obvious server side issue have different results between browsers?)

I changed all permissions in the upload directory as well as the c5 directory to 777, and have also verified my php.ini settings (which seem to be the same across all the servers). Just for reference here are the important ones:
max_execution_time => 0,
max_input_time => -1,
file_uploads => On,
upload_max_filesize => 32M,
post_max_size => 32M,
memory_limit => 128M,

No suhosin or SecFilter variables. I have root access so I can change any of the settings but I need to be careful it is on a production server. Any help would be greatly appreciated, again the weirdest thing is that it works in IE8 but not others (some kind of client-side issue interfacing with the server possibly, but why would my other c5 sites on different servers be working ok?) Thanks!