50% of full size pictures will only show after page refresh

Hello! I'm using Joomla 1.8 and Gallery 2.1.2 for my website. Alle the thumbnails in the albums are perfect, but when I click on the thumbnail and want to see the full picture.. in 50% of all situations I see an empty page! All the info around it, like filename and buttons are displayed, except the picture... Only a refresh will help to see the picture.

mod_rewrite is disabled, there is enough memory on the server, the problem doesn't exist in the standalone Gallery, so I really don't know how to solve this.