There is no problem with your GS911 WiFi
I am afraid you are still misunderstanding the product, what you are trying to do in using the PC app via WiFi is not the way the product is designed.
There is no problem with your GS911 as your own posts clearly show
"It works perfectly using the USB cable - no error messages, not a single one."
Correct, the PC App is designed to work only via USB with either GS911 WiFi or the earlier GS911 versions
"It works perfectly in WiFi mode and Open browser for selected feature - no error messages, not a single one."
Correct it is designed to only use the browser to work with it in WiFi mode, the functionality (the App if you like) is provided by the firmware internal to the GS911 WiFi with maintenance functionality via the cloud
"I get countless error messages when using the GS-911 program in WiFi mode."
Correct, the PC App does not work over WiFi connection it is not designed to work that way nor should it be designed to operate that way, it is pointless to do so as it only allows for Windows PC's
The WiFi Utility has several functions
1/ Perform initial set ups including configuring the PC App to use USB
2/ To aid people in finding the GS911 WiFi where they have not set up reserved or fixed IP No's
3/ To perform firmware updates while the GS911 is connected via USB