This happens when you import your proxies in wrong format. Most of the tools use this format to import proxies - ip:port:username:password
All fields are separated by colon. WAC, however, uses this format - ip:port;username;password
Please ntoet hat before username and password there is a semi-colon (;)
Make sure you import your proxies in this format or you will get a message saying "running your scraping threads"
If you are still getting the error (even after correct format) then your proxies are not working. You might think GSA shows them as passed but remember GSA only checks google.com against the proxies. I use scrapebox to do a custom search and actually scrape google.com (google.com/search?q=) and the proxies fail. So even though GSA shows them as pass, it does not use it to scrape anything.
If format is correct then proxies are not working. Ask your provider to change them or contact me to do a check for you.