OneDrive for Business stops syncing, window stuck "Logging in" all tried, MS support helpless

A full uninstall, reinstall and preview build that all behave the same, on a new PC, with the sync fine for the first couple of hours, points away from OneDrive itself. An endless "Logging in" usually means the sign-in token cannot be refreshed, so the cause sits in the Windows identity layer (the account broker, the device registration state, the time or the network), and OneDrive just shows the symptom. I would check these on that PC, in this order:

• Device registration and token state. Run dsregcmd /status in a command prompt as the user. Look at AzureAdJoined or DomainJoined, and under SSO State at AzureAdPrt (it should be YES, with a recent update time) and the diagnostic data and any error code shown. If the PRT is NO or the update time is old, the token refresh is failing, and that is your root cause.
• Event Viewer. Applications and Services Logs > Microsoft > Windows > AAD > Operational shows the token and broker errors with a code, and the OneDrive logs (in %localappdata%\Microsoft\OneDrive\logs) will line up with the same times.
• The account broker cache. Sign out of OneDrive, then delete the contents of %localappdata%\Microsoft\TokenBroker and the folder %localappdata%\Packages\Microsoft.AAD.BrokerPlugin_cw5n1h2txyewy, and remove old entries in Settings > Accounts > Access work or school and in Credential Manager (anything starting with MicrosoftOffice16 or OneDrive), then restart and sign in again.
• Time, TPM and network. Make sure the clock is correct and syncing, run Get-Tpm to check the TPM is ready (token keys are stored in it on a new Dell), and see whether a proxy, VPN or TLS inspection is touching login.microsoftonline.com and the OneDrive endpoints.
• Entra sign-in logs. For that user and time, look at interactive and non-interactive sign-ins. A failure code there, such as a Conditional Access block or a device state problem, would explain a stuck login that the client cannot show.

To narrow it down quickly, test the same user on another PC and another user on this PC. If the problem follows the PC, a fresh Windows profile or a reset of the device registration is usually less work than a full Windows reinstall.