Probably worked together with 'deleting the mempool.dat file' since it also contains that transaction.

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I know this is late but if it's just an issue with a single unconfirmed transaction,
a simpler solution is to create a signed transaction that can be broadcast outside of Armory.

After using "Clear Unconfirmed", create the same txn using the cold-storage method but instead of using Armory to broadcast, just copy the the "RAW TX (HEX)"
and broadcast it using an online service like: https://live.blockcypher.com/btc/pushtx/ or https://blockstream.info/tx/push

If you don't know how to get the Signed Raw Tx, follow this:

• Click "Send", fill up the transaction info: fee, recipient, amount, etc.
• On the same window, check "Create Unsigned", then click "Continue"
• On the next window, click "Copy to Clipboard", then "Continue"
• Next, paste the copied text, click "Sign", "Continue" then type your password
• Click "Copy Raw Tx (Hex)".
• Lastly, go to an online push service and paste and send that signed RAW tx.

After getting one confirmation, the conflicting transaction in your mempool or Bitcoin core's wallet.dat will be dropped.