Problems logging into PSN

In the spirit of xkcd 979, let me describe something that recently happened.

My PS4 could not log into PSN, the PlayStation Network. It would show various error codes:

NW-31473-8
CE-38599-4

Then I logged out, and tried to log in by inputting the password. I saw a new error code:

NP-40831-6

I tried logging into PSN from my laptop, from the same home network. I saw:

Error
This page can't be displayed. Contact support for additional information.
The incident ID is: N/A.

Sometimes I’d see a long incident number. It would change with each attempt.

I called the PlayStation support center. They didn’t have any information about the error codes I saw. They checked that my IP address was not blocked. They suggested setting up port forwarding. I thought it would be an odd way of dealing with the logging in problem.

I rang up my ISP. The support person said that there’s a problem with my broadband connection and they’re going to send a technician. The technician detected that some cable outside of my house got rusty and the signal level went down from -1dB to -11dB. The technician fixed this. I waited maybe 3 days and my PS4 started logging into PSN without problems.

So, what was it? Why a weak broadband connection would result in PSN refusing logins from my home network? I don’t know! I could speculate but I have so little information that I would be almost certainly wrong.