[Gelöst] WPML can't run normally. There is an installation or server configuration problem

Background of the issue:
I am trying to check my WPML installation because it gave an error message. The issue occurred when I accessed the URL: versteckter Link.
Backend login information
username：mayingjiu
password：Zt0!E#%!qg

Symptoms:
WPML can't run normally due to an installation or server configuration problem. I received an AJAX Error: error [0].

Questions:
What is causing the AJAX Error: error [0]?
How can I resolve the installation or server configuration problem with WPML?