Try the following couple of steps and check if it is working.
1. Click on the menu at top right and click 'New incognito window' (Ctrl+Shift+N). This will open a new window. Then try use the mindmap maker app and save it in Google Drive. If this step is not working, check the following way.
2. Click Menu (at top right). Then click 'More Tools->Clear browsing data...'
Click 'Advanced' and then select 'Time Range' as 'All Time'. Select all options beneath it. Then select 'Clear Data'.
Then try use the mindmap maker app and save it in Google Drive.
If both steps are not working, let me know.