Looks like there are some z-index clashes. I have applied some fixes for you by adding the block of css codes below into /templates/jsn_glass_pro/css/custom.css

[gist]
body #mm.layout-full {
z-index: 99999;
}
[/gist]

By the way, there is something not right with your site's caching strategy as I seem to keep hitting the following error when trying to login until I clear your Joomla cache,

Die Anfrage wurde zurückgewiesen, da der Sicherheitstoken ungültig ist. Bitte die Seite aktualisieren und es dann erneut versuchen.

I would suggest that you do something with your caching strategy as this would push your users away.

Thanks @ Mark, the caching problem I had already mentioned here. I was told that I should increase the cache time. Now stands on 1 week. However, the problem sometimes occurs.

Do you have an idea how I can eliminate this?

All right. Is already planned.

Thanks