Hi Timur,
There will be nothing here that you haven't thought of already, I'm sure, but here goes:
- Does this also happen with an older browser, or with IE in compatibility mode?
- The URL shown seems to be pointing at sitemods/html - can you try reverting to the out-of-box popup_frames.html?
- Compare the contents of site\mods on the secondary with the primary?
- What does that little icon at the right-hand end of the Chrome error bar mean? (I can't make it out). Can you run Wireshark or similar to see what is being requested and returned? Compare that with what you see on the primary or on the other secondary?
As a 'Plan B', do you have the resources to configure another secondary to replace the one that is apparently faulty?
Hope that helps...
Regards,
James