I did same as ACVEER mentioned, worked okay (Chrome on Windows 10). Just make sure to open the activation URL link from the licence payment email in the same browser as you have Tabs Outliner extension installed on and have the extension enabled, and its window open too. 

Okay, I figured out that to restore the paid licence 'pro' features, you need to go back to the acknowledgement email and open the link URL there in the same browser that you have Tabs Outliner installed on. 

Seems to work fine again now, on the version with the modification of the "options-backup.js" file. :)
 

Noticed the mention in original help files: 

• Links can be opened directly from Backup View with Ctrl-Click (Middle-Click) or Shift-Click, as Backup view is just an a regular HTML page. (This shortcuts also works in Main View).

Wow, it worked! 

Thanks so much for finding the workaround for this @566-eliio ! 

Any idea how to regain the pro licence features (which I paid for, but which now no longer seem to automatically turn on when I access my Google Drive account)? 

[ P.S. I might add, in your final step instructions, maybe to write the final detail about selecting the folder: 

6. ...click "Load Unpacked" and browse to the modified version of the "1.4.134_0" folder (with the replaced options-backup.js file in the \backup\ sub-directory), and click "select folder." ]