Starting around SSMS 18.x, Microsoft changed the way credentials were stored. This caused any saved passwords from previous versions to stop working in the server connection dialog popup.
To fix this, just delete the servers from the drop-down list. Alternatively, exit SSMS, clear out the saved servers in C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\18.x\UserSettings.xml and then restart.
If you use registered servers, the saved passwords will still work and will automatically be saved into the popup. If not, you'll have to type them in again.