90,001. what is it exactly?

As of lately a lot of my friends have been getting booted constantly with a 90,001 error. Does anyone know what causes this, what fixes this, or any information at all as to what could be the culprit? At least one of them is nearly ready to quit the game, they're so bad, and her pc is brand spanking new, new modems, new router, new cables, but still persisting to have this same error over and over.

network connectivity lost.... more often than not, it is a stability issue along your route to the server. But, it could also be tripped up by more local issues--local network device freaking out (or local security tripping up), edge of signal or other environmental issues affecting quality of the signal to your local head-end, oversaturated lines in your neighborhood (or a bad device upstream from you injecting noise on the line), oversatured exchanges at any of the dozen or more hops between you and SE... it can be quite a laundry list.

Basically, you'll need to verify stability locally, to your ISP's gateway, and then beyond. One of the more common ways is to simply run multiple tracert's to the server and look for major swings in the response times. Usually taken as an indicator of congestion upstream from your location, but it can also crop up at your first few hops if there are signalling issues nearby. IF you go this route, also try to get snapshots of your modem's signal and error logs, round up all the details, and email them to your ISP's Tier3 support. They will be the best people to have conduct a proper investigation into any potential issues, and then request any needed escalation for correction.

Alternatively, you could try a VPN/Proxy service and try to adjust your routing through different regions to see if you can get a more stable route. If you do...it is a pretty dead give-away there are routing issues with the path your ISP has assigned and you NEED to pull them in to get it addressed. The fact that you can correct it via VPN/Proxy is pretty damning evidence to support your case with them to do something about it.