How to understand why the page of the local server does not open? (Endless loading)?

Question

proxy

please correct the tags i could be wrong.

spoiler

This kind of problem is intermittent. With any webpack builds, the essence is the same, when the project is launched, the page loads endlessly, while the page that is proxied from another server opens normally. I don’t even understand how to debug it, because there is nothing in the console, in browserSync too.
The problem is solved by itself after a while (Now it was 5 hours, the computer rebooted twice)
How can I understand what is the reason?
server openserver apache 2.4,