Fatal error when integrating NextGen-Gallery

Hello!
I don’t know how to correctly search for this problem. All keywords I tried showed no results. So please, if there is already a solution to this problem, give me the link!!! 🙂

My problem:
Today, I wanted to add a NextGen Gallery-Slideshow to a new page of the wordpress-site that I am the administrator.

Usually would open a screen/dialog where I can choose the gallery and the displaying options.

This time the screen shows my frontend page with at the bottom this error:

Fatal error: Call to undefined function get_current_screen() in /homepages/17/d96476071/htdocs/international/wordpress/wp-includes/admin-bar.php on line 486

I found a thread here in the Forums that says it could have to do with a e-commerce-Plugin. I deactivated the plugin and nothing changes. I even applied the change suggested by a member to the file of the path above – no change.

I am using TheCartPress for the e-commerce section on my page. I’m going to upload the products of our company, but I firstly want to sort out this problem, because the slideshows are more important to us than the shop.

100thanks to everyone who can help or at leat link me somewhere or give me keywords so I can search on my own.

Assilsan