Sync's WebUI is protected with login/password, that you define when first opening WebUI. It's not your user account's password and it's not related to any other accounts that you might have. This is just a protection against unauthorised access to Sync WebUI solely.
If you've lost/forgotten your WebUI password, you can manually remove login & password settings in the Sync storage folder. Please note that it will lead to device duplication in the "My devices" list.To reset login and password settings, follow these steps:
1) Quit Sync and close WebUI.
2) Go to the Sync storage folder.
3) Locate and remove the following files:
4) Once the files are removed, restart Sync, open WebUI and enter your new login and password.