I only get this message with Google Chrome,
both on my fixed computer and on the laptop.
When I approach MS Edge, I do get the desired result.

Strange but true, this problem does not occur on my smartphone
and on a very very cheap tablet (Kitkat 4.4).
Both Android. Here also using Chrome.

Calling Logo is done via LAN.
Anyone any idea where things are going wrong and
in which direction I should look?

Hi,

start Chrome. If you see the error, press CTRL & F5. I use Chrome since years, no issue together with LOGO!.

Hello,

Maybe this topic will help you. LINK

Thanks for the help, but I already posted my problem there.

Hence this new topic. 

Not badly intended but it remains the same. Error remains.
As I already wrote: no problem with the smartphone 
and that tablet that you can't really do much with.
Knowing this I will keep using MS Edge to keep LOGO! to read.