I ran into the same problem and found a fix. You must tell your keychain file that the Cornerstone application is suitable for using the keychain file (do it manually).
First go to Utilities β Key Access.
Secondly, find the key key file. There must be a login key, and the name must begin with "Cornerstone".
Thirdly, right-click the keychain file and click "Get Info."
Fourth, in the window that appears, select the "Access Control" tab.
Fifthly, click the switch that says βAllow all applications to access this item.
Enter the password, save the changes.
Sixth, go back to the cornerstone and repeat your username and password for the repository. Save the changes.
Seventh, commit the changes you were trying to commit.
Eighth, leave me some love at Lawsonry.com. =)
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