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Does the camera fail with an FFA error displayed on it, with both/either lens assembly mounted on it?
If so, then what shows on the camera when it is powered up without any lens assembly mounted on it at the time of power-On ?
OK, that's good news.
Try cleaning the contacts on the lens mount of the 28-105 lens assembly, wiping gently with a clean dry soft cloth, gently so as to not remove the thin delicate coating on those contacts.
Then try that lens assembly again.
Let me know the result please.