403 Error is "Access denied" and means that you've to autorize your proxy with your home's IP. So, if Scrapebox don't find anything there are 2 problem:
- you have not autorized the proxies
- your proxies are banned from Google
In the second case wait few hours after hard scrapind and go harder the next time. Actually I'm using Scrapebox with 100 private proxies and works good, you've only to check it.