Charging is not supported with this accessory fix?

I have a iPhone 3GS, and I have a very strange problem. There's nothing pluged in, yet the lock screen is replaced with exclamation mark, and "Charging is not supported with this accessory" is written below. I cleaned the port already, with no result.
When I connect my phone to PC, the "charging is not supported with this accessory" message appears and disappears all the time.

I have the original charger and cord.

But when I plug the phone directly into the charger, it works.

Oh, and my iOS version is 6.1.3.

Is there anything I can do about that? Because that exclamation mark is starting to annoy me…

Here is some things to try http://www.gatorspit.com/tips/iphone-wont-charge.html

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