I finally found a solution with KDE 6 that solved the problem for me.
- In the Application Launcher, search for Mailspring.
- Right click on it and select “Edit Application…”
- Navigate to the “Application” tab.
- Update the “Arguments” field to read
'--password-store=gnome-libsecret %U'
- Click “OK” at the bottom and close the menu.
Hope this helps anyone else encountering this issue in KDE 6.