Marc found this interesting note in the FAQ section of 1 & 1 :

Can the memory usage limit be increased on shared hosting with a php.ini file?

Due to resource limits on our Shared Hosting machines, it is not possible to allocate more than 20M of memory to PHP, although phpinfo() may report a higher number. You will be unable to increase the memory usage limit with a php.ini file.

Very cool note Especially when you are check the memory limit via your PHP script. Time to change the hoster…

These users are not able to generate thumbnails (the script suddenly stopped) or get a memory failure message. The phpinfo() returns a memory_limit of 40 MB, normally enough for a 1600x 1400 resolution in a WordPress environment.

Thanks to Nana Torres, I’m able to debug this on a 1&1 web server and after 5 hours I found that imagecreatefromjpeg() stopped if you require more than 20 MB. This happened very early , when you uses a larger images. UPDATE : See FAQ

You can test this on your machine, with the attached script. If you see at the end a “*** SCRIPT END ***” your are not having the troubles…

Download 1&1 Test script
Simple PHP script

Update : See comment of Philip D. , PHP5 seems to do the work…