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.
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Open the synchronization settings
Open Sync setting -
Choose "Link passwords" to ensure that your account password and encryption passphrase are the same.
Link Passwords
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.