Platform: PC (non-Steam)

Error code(s): 90002 error is the only one I've seen when this occurs

Type of connection: Wired

Other programs running: Steam, Blizzard battle.net client, Firefox...which makes me think that maybe having FF open could be a factor because I swapped to the new quantum version that came out recently. Prior to that, I used Chrome, so I'm going to swap back to Chrome just to see if that makes any difference (color me skeptical that it does)

Frequency: It's hit or miss, which makes it all the more frustrating. One common factor is if I'm in/near large groups of people in combat, such as out fighting A/S rank hunts. I've also had it drop me while in my own house when I would go to check retainers (small house, not a whole lot of furniture/stuff in there). Had a Frontline roulette put me in Secure for the first time ever 2 weeks ago, only to have the 90k error (and subsequent disconnect) 4 times in a 5 minute span.

Booted to after disconnect: Start screen

Everything I've done: Swapped network cable from my PC to my router, rebooted router + modem, directly connected my PC to the cable modem (thus bypassing the router), rebooted PC multiple times. I've had some Windows updates come through during this timespan, but the issues I've had started before those updates were installed. Updated video driver to latest version to no effect (and no surprise there, really). Only other thing I haven't done is check locally with other people I know game, but I'm the only person I know of in my town that plays this game (which is a totally different "problem", heh). Other firends that I do have playing the game (and not having these issues) are on different service providers; ideally I would find someone on same provider as I am, but haven't had any luck with that. As noted in OP, tracert that I've run dozens of times since this started have not once had any drops while the traffic was within my ISP's network.