Hello!
The Page-loading-times have now normalized again, but to the other problem:
I created an Test Account:
Username: testaccount
Password: testpassword
edit: sometimes the problem just happens when one chat is already open, and you want to contact another “salesperson” in my case.
edit2: you have to click at “Anbieter kontaktieren” on the right sidebar for the shortcode
[ic_open_dialog id=13 name="Anbieter Kontaktieren" class="class_btn_msg"]
thanks already for your Support!