Impossible uploading certain pictures, I get [Error id: 2500 the reason is: S3Upload(19)]

Hi,
I am experiencing a quite weird issue.
I have no problem uploading pictures that get resized but when uploading smaller pictures I get or a continuous upload or an error [Error id: 2500 the reason is: S3Upload(19)].
As I work from corporate network I thought of some firewall activating but that would not explain why bigger files would not get blocked.
I really need help!
Thanks,
Marco