Seems like you're not the only one who's getting that "Key Error" message;
This "send Crash Report" issue suspiciously looks like the same issue as yours: https://github.com/spesmilo/electrum/issues/6394

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Try to find the private key from the wallet file itself, but that requires you to unencrypt the wallet.

First, disconnect from the internet or use an air-gap machine:

• 1. remove the password of your wallet in 'Wallet->Password' type your password and leave the new/confirm password fields empty.
• 2. Then find your wallet file in the Electrum data directory/wallets, and open it using a text editor.
• 3. Lastly, find "keystore" and below it, there should be "keypairs" where you can find the address after its private key.

If you found seed and master keys along with your imported addresses, there's really something wrong with your wallet file.