Hi,
I still have the same issue, but I have investigated further.
Following the user guide (http://www.zenphoto.org/news/problems-with-albums-and-images), I have added the debug parameter at the end of the image URL used to generate the thumbnail, and I have the following error :
------------------------------------------------------------
Image filesize: 6447963
Debug i.php | Arguments:
size = 800
width =
height =
cw = 0
ch = 0
cx =
cy =
quality = 85
thumb =
crop =
Fatal error: Out of memory (allocated 84410368) (tried to allocate 5184 bytes) in /homepages/[...]/photos/zp-core/lib-GD.php on line 91
------------------------------------------------------------
So it seems that real PHP memory limit is around 80M, and not 120M as displayed in the ZenPhoto back-office. I tried to increase the memory limit thanks to a php.ini file, but it seems it doesn't work, even if the 1and1 documentation is saying that it should work (https://assistance.1and1.fr/hebergement-c65619/programmation-c65626/php-c65653/creer-un-phpini-a703435.html). => I will contact my hosting provider and ask for some support.
Other point, my image has dimension 5184 x 3456. I tried to reduce the size of the image step by step, and the thumbnail is generated for an image 4941 x 3294 (and below, of course).
So at the end of the day, if I can't increase the PHP memory limit, I'm afraid that the only solution I have is to reduce the size of my images. The very weird thing is that the thumbnails were generated correctly with php 5.2 for the same image size, and now with php 5.5 it doesn't work anymore... "Please migrate to php 5.5 for better performances" said my hosting provider :-)
Regards.