Here I describe a solution that works from time to time when the DeFi wallet does not start correctly. Sometimes even this approach does not work. First, please make sure you have a secure backup.
Go to the .defi folder and delete the contents (or move the contents to another location)
If you can't find the .defi folder, this text may help: Show hidden folders
Then restart the computer and open the DeFi Wallet. Partly this trick works. If not we can only recommend to delete everything (except backup) and reinstall the app.