Usually this happens because you have Omnifocus configured to use a distinct encryption passphrase from your Account password. While this adds more security (and prevents Omni to be able to decrypt your data) this requires you to remember the 2 different passwords (account password and encryption passphrase).
If you dont remember setting up a separate encryption passphrase then please ensure that in OmniFocus the encryption passphrase is linked to the account password.
Open the synchronization settings
Choose "Link passwords" to ensure that your account password and encryption passphrase are the same.
Note that FocusGTD does support a separate encryption passphrase so if you remember both passwords then you should be able to use distinct passwords.
For more information about sync passphrases check the OmniFocus documentation.