Okay, so after:
- Updating my IE settings
- Setting the game to run on Windows 7 compatibility
- Setting my router to forward the appropriate ports
- Updating my graphics card software

The only thing that has worked is running a VPN service on my computer, as a detailed comparison between my computer and my husband's computer revealed that he was running a VPN and I was not.

A Reddit commenter on this same issue blamed the problem on a particular internet node in San Jose. I live in that general area, and when I set my VPN to route through San Francisco, I still got 90002 errors, though fewer than before. But I set my VPN to route through Houston Texas a couple of days ago and haven't gotten a single one since. Coincidence?

I doubt this is due to anything on our own computers or networks.