Configure Network Connections > Wi-Fi > Wi-Fi Security. I have the option ‘Store password for this user only’ checked. Now when entering a new password, the old password is stored instead. WORKAROUND: Choose ‘Ask for this password every time’, connect once using the new password, then again choose ‘Store password for this user only’, type the new password and click OK. Now the new password is stored.
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Tested again in Plasma 5.24.7 and the problem seems to be solved. Thank you!