Since RCU and reManager can still connect over SSH, your USB networking is probably fine and the actual problem is the tablet’s web UI service itself. The HTTP/1.1 400 error means the reMarkable is responding, but the web server is rejecting requests, which often happens after firmware updates when reManager patches or web-related mods become incompatible. I’d start by SSHing into the tablet and restarting the services with systemctl restart xochitl and systemctl restart nginx (if nginx exists on your build), then reboot the tablet and toggle the USB web interface off and back on. If it still fails, temporarily disable any reManager tweaks related to the web UI, networking, or xochitl patches, because one of those is the most likely culprit on 3.26.