HTTP error when uploading an image

Hi guys, I have a serious problem with image upload. Every time I want to upload en image I get HTTP Error – that’s it. Nothing more, no specific error or explanation, nothing. The version that I am using ist 4.1. I have already tried:
– PHP 5.4 and 5.6 (default was 5.3)
– different image sizes and types
– different pages (all 4.1) and different hosting providers
– without any plugins and with a default theme
– different browsers (except IE, I’m a Mac user)

I desperately need a solutions because I have sites that are online for some time now and until now it was everything OK, after last update all my WordPress sites have the same error and my customers are, as expected, not very happy and neither am I.

Thanks