Something's not right in the header upon logging in

Hello there, I’m in dire need of help, and I hope somebody could help me solve this. There’s no problem with the site’s homepage before logging in, but after I log-in (both admin and user log-in) the header (particularly the navigation) goes like this:

by the way, here’s the link. Since I don’t know how to add image here, Just base my problem directly on the site.

Click here!

I created an example user account for you to be able to look at the home page upon logging in (login button at the uppermost-right part of the page).

Here’s the details:
username: user
password: userresu

I do hope somebody could help me with this. Thank you 🙂