Then that error message is weird... but there is another way to nuke the wallet and recover the previous one with the seed.

1 - In "About" click on "Show Sia folders" and in the folder that pops up go to "sia" (alternatively, the folder path in Windows is C:/Users/your_user/appdata/roaming/Sia-UI/sia).

2 - Close Sia completely (in Windows, click close on the system try icon)

3 - In the folder you open, delete or rename the "wallet" subfolder. As long as no folder with the name "wallet" remains you are good

4 - Open Sia again and and click on load a wallet (the icon is an old key, iirc)

5 - Use your recovery seed when prompted. Be patient, the re-scanning of the blockchain can take something between 10 minutes and 1 hour