A yellow sign replaces the project's icon in the Projects list, and clicking it explains the cause, for example FTP sign-in rejected: the password saved in SyncNow is wrong or has changed. Next to the project's name the state says Authentication required when credentials are missing or rejected, and Offline when the server cannot be reached; while SyncNow's FTP connections to the hosting are refused or paused, the Overview also says Hosting connection is failing under NEEDS YOUR ATTENTION.
Click the yellow sign to read every failing connection and when it failed. Other reasons include a wrong password or a host filtering the connection, an FTP server that cannot be reached, a revoked credential, a credential pointing to a different server or folder, a saved password that cannot be read and must be saved again, writes paused by the hosting, or a database connection that fails.
Open the Connections tab, or click View next to Hosting connection is failing in the Overview, and click Edit on the Hosting card: a password kept by SyncNow shows as Stored securely, with Replace to type a new one, Test connection tries it, and Save hosting connection tests the connection before saving. The sign goes away by itself as soon as an operation on that connection succeeds, and projects nobody is working on are rechecked about every 30 minutes (every 10 while a problem is open) with a check that only reads.