Hy,
Did you found a solution for this?
I run into the same issues and couldn't specify the reason for this.
Only the NextGEN Gallery is used in my Shop, too.
I have deactivate this plugin, but this don't solve the problem.
The only solution I found, is the deactivation of "Add to cart Ajax". Without this option the customers could add products to Shopping cart in 3.5.
I have to rollback to 3.4, until the solution is found.
Stefan