After multiple attempts to uninstall, reinstall, repair, and reset Windows Sandbox and other system components on Windows 11 (Build 10.0.26200 / 25H2)—including repairing the OS itself and uninstalling/reinstalling several Windows updates—I was finally able to pinpoint the root cause.
In my case, the issue was graphics driver–related.
Initially, I updated my discrete GPU drivers, but that alone did not resolve the problem. I was running an AMD Radeon card, and even with the latest drivers installed, Windows Sandbox continued to fail with the Reconnect issue. I rolled back all AMD drivers, ran the AMD Cleanup Utility, and saw temporary improvement, but the problem eventually returned.
What ultimately resolved it was disabling the onboard graphics adapter. After disabling the Intel UHD Graphics 770 driver in Device Manager, Windows Sandbox began launching normally and consistently. I then reinstalled the latest AMD drivers—specifically the OEM drivers from the Dell support site, not the AMD Adrenaline Suite—and the system has remained stable since.
TL;DR: Before repairing or resetting Windows Sandbox or other system components, try disabling the onboard graphics adapter in Device Manager. This alone resolved the issue for me without requiring further OS-level repairs.