I really appreciate your responses.

1. The version of bitcoin-qt is the latest 22.00.
2. I have backed up the wallet.dat
3. Blockchain scan is complete
4. On any block explorer (such as blockchair.com) the transactions are correct. The wallet MUST have 0 btc.
5. Only incoming transactions are displayed on bitcoin-qt. So the balance is positive, but it should be 0.
6. I tried to send a transaction, but it never gets sent on the bitcoin network.

The address to check is this one: 13Sfy3ucX8yCJkLQeALohX3K3PhtJhdCZL

I'm going to try a rescan now and see if everything works out.