I know this is probably a long-shot, but I’ve had a few times where my certificate issues get resolved by doing this.
Start Xcode. Select Xcode->Preferences and choose Accounts.
Select your Apple ID and click the “View Details” button.
Click the Refresh button in the lower left of the dialog sheet. Wait for it to complete… and click it…again.
This procedure seems to sometimes refresh things in a way Xcode likes. Maybe worth a shot unless you’ve already tried…