General settings freezes after applying changes – Version 8.0

At a post I started earlier the solution was to update my comparison theme to the latest version. I used version 6.6.3 and now I upgraded it to 8.0
Everything works just fine, except one thing. When I make a change in the general settings page, for example I disable breadcrumbs, and I hit apply. The website keeps loading and won’t do anything. When I try to reopen the site (back-end & front-end) it won’t load. It stays at a blank screen. When I restart my browser (Firefox) I can open my site. but the changes weren’t applied.
Things I tried:
– searching this forum (but I didn’t found one topic about this problem)
– I tried another browser (Chrome), but the same problem occurred.
– Reinstall the theme (I thought maybe it was an installation error)

Maybe it’s a corrupt file, but I don’t know witch one. Or maybe it’s some settings from my browser/computer?

Anyone any possible solution?

– Jorrik