How to determine what kind of page crashes to the user?

Question

HTML

Good day.
Sorry in advance for the most likely non-obvious question, but there are no more ideas how to best form this unusual problem.
Going straight to the problem's source data set:
• There is one piece of software that maintains a connection to a remote server by sending periodic requests and receiving a response.
• There is one user who sometimes (usually after 10-20 minutes after the start of the software) in response, instead of the usual response from the remote server, a snapshot of the html page arrives with the title "Initial setup".
Based on what I understood from the page code, isolating the words etype (Etype provider) and the user's place of residence (Kaliningrad) - this is one of the provider's pages.

As I understand it, periodically for some requests (obviously not for all), a page with the heading "Initial setup" comes in response and thereby interrupts the connection of the software to the remote server, since the software considers that the server is unavailable and closes accordingly.
After lengthy questions from the user, the case did not move forward a single step. The user assures that he has never seen this page during normal surfing and does not know what he is talking about at all.
What kind of page is this and how to stop its sudden appearance?
In which direction to dig? Could there be a problem in the router (but why then the page is completely tied to the name of the provider, and not the brand of the router)?
I have already advised calling the provider, but still I am looking for a solution on my own.
I would be grateful for any advice.

Answer 1

Ask the client to install fiddler and dump the entire application session so that all requests are visible. It is possible that the provider does not work out DPI correctly, then it is possible to understand which requests cause such a redirect.

Answer 2

Is it stupid to save this page exactly as an html file and view it in the browser without reading the code, is it too difficult a task? :)
And in general, if it is clear that this is a provider and the provider is known, you should not ask on the toaster, but at the provider!
The reasons for the appearance may be ... well, not a dozen, but certainly more than three, it’s not very meaningful to describe them all, it’s easier to initially strain the provider, let him explain which artist solves the client’s problem .