HTTP-Error when uploading images

When i try to upload images i get a HTTP-Error (Just that, no more info)

If i disable the Plugin, file upload is working fine.

There is nothing specific in the WP error logs about this.

I have this problem on two different sites, and also on the test enviroments which are running on different servers.

I tried two fixes from the Forum, but they didn′t help (One functions.php fix and the .htaccess fix)

The only thing i see in the error logs is something about w3total cache. Could there be a compatibility problem since they finally (yesterday?) updated their plugin?

Please help.

Thx.