NOTE: This post discusses issues logging into DCS World multiplayer and managing the multiplayer database of serial numbers. It has nothing to do with purchase records and StarForce. If you have issues with activation or purchases, please submit an ED support ticket.
Trying to log in to the Masterserver for some DCS World multiplayer action and getting an 'Invalid Serial Number' error? Here are some things to try:
1) Ensure you have valid serial numbers for the DCS modules you own.
Go to the DCS website, log in, then click the 'Payments' button near the bottom of the left column of the page. This will list all the products and their serial numbers that you have purchased with that account. If, for some odd reason, you have multiple DCS accounts, you'll have to log into each to check all your serial numbers. I strongly recommend only having one DCS.com account or, at a minimum, only buying your games through one account. This will save you headache later. Haven't bought the product yet? Go to the DCS E-shop and pick it up!
2) Make sure the serial numbers are all bound to the same account.
When you login to the ED Masterserver or resister an account through Multiplayer, the serial number of the product being used gets registered (aka 'bound') to the login info you used. When the DCS products were standalone, you could use a different Masterserver login for each one and still access multiplayer without problems. With DCS World, all your products need to be bound to one single account. If they aren't, you'll get the dreaded 'Invalid Serial Number' error and you'll need to unbind numbers from your other accounts. Go to the ED Serial Number Management website and log in with the same DCS.com account you used to purchase the products. You'll see a list of the serials that are bound to that account. If you don't see all your products it either means you haven't bound them yet (see step 4 below) or they are bound to a different account. If you have other accounts, check them to see if they are bound there. You can also unbind serial numbers from other accounts on the same site. Once you've got your serial numbers unbound from your other accounts, go log in to Multiplayer and log in with your DCS.com account to bind your products to that account.
3) Ensure the correct serial numbers are activated.
Start DCS World, then, while holding the shift key, start multiplayer. Keep holding the shift key down. A serial number activation window will pop up. Verify that number in the box matches the serial number you have for that DCS module listed in the window title (see step 1 for getting those serial numbers), then click Next. If the product is already activated, a dialog box will pop up, asking you if you want to continue without using another activation. Press 'Yes'. Whichever path you go, be sure to keep the shift button pressed when you click the button, and continue to hold shift down to bring up the activation window for the next product. Repeat for each product. If you don't see an activation window for a product that you know you own, it either hasn't been installed to DCS World, or there was a problem with the installation. Re-install it and try this step again.
4) Log in to the Masterserver with your DCS.com account to bind the serial numbers to that account.
Use the same login information as with your DCS.com account. You don't need to register the serials to the account separately anymore - this will happen automatically when you log in.
5) Still having a problem?
Submit a support ticket at this site and the folks at ED will get back to you as soon as they can!
Serial Number Management: http://www.digitalcombatsimulator.com/myserials.php
Submit an ED Support Ticket: http://support.lockon.ru/en/
DCS E-shop: http://www.digitalcombatsimulator.com/index.php?end_pos=1322&scr=shop&lang=en
DCS World download page: http://www.digitalcombatsimulator.com/in..uct&lang=en
DCS modules download page: http://www.digitalcombatsimulator.com/index.php?end_pos=136&scr=products&lang=en

Shoot to Kill.
Play to Have Fun.