As advised by AWA we now have contacted the AdWords Support Team.
Just noted a similar thread [webmasterworld.com ] from last year on the same issue quoting
We get "403 Forbidden" error when we try to make more than 20-30 requests, even using regular AdWords customer account. Google locks access to Keyword Sandbox for about 30-40 minutes after this message.
Are there still ideas to handle this problem?
Sure, Google has to protect its tools from being massively misused by automated software - that's completely okay. But it should not prevent the ordinary manual user from carefully selecting keywords for an extended AdWords campaign. It definitely needs more than 20 requests to do it thoroughly!