Create backup iPhone with forgotten encryption password
The short answer is no. The phone thinks you are a thief trying to steal the data and won't cooperate with attempts to initiate a second backup. Your best bet is if you have a computer that was syncing / backing up the device and follow the steps listed for a disabled device with a passcode:


iOS: Device disabled after entering wrong passcode


Once your iOS device is locked with a passcode, the data at rest is scrambled and the decryption key is only stored inside the iOS keychain or inside a backup of the device in iCloud or on a computer.

Even if you connected the device to a computer and got past the hardware telling the USB storage to not present itself - you would need to perform a decryption of whatever forensic image you retrieved.

Without knowing your situation (exact device, iOS version, whether it's an older device that doesn't have Data Protection, where else you may have backed up the device so that you could try restoring a backup onto an unlocked device (where you could then choose a different passcode) it's hard to guess what your best move is next.

You could:


edit more details into the question
ask a follow on question once you've explored your options
Contact Apple to do some troubleshooting / learn more options