sometimes with the g2 block image enabled, I have broken images in these block

It's not all the time but sometimes when I refresh the page I have a broken image with the thumbs in the block image.
When I look at the html code, the code is correct
I have an install of gallery2 in a folder named "albums-photos"
And I repeat it's not all the time but sometimes

I have installed an another gallery2 on an another drupal website and I have sometimes too these strange behaviour.
On the site I have installed square thumbnails module for gallery2 and I use a customized folder for the install (albums and albums-photos)
The block image shows random images in the right side on my homepage.
I have experiencied some problems with url rewriting and I have desactived url rewriting in gallery2 but keeping it for the two drupal sites.