Updated: The issue has now been resolved!
If you have purchased a DLC on Steam between July 12th and July 20th, you should now receive it after you log in to your Heroes & Generals player account with the same Steam ID you used to make the purchase. Note: you might need to log in twice for the DLC to be redeemed.
Outdated:
We are receiving reports that some players are seeing a "server error" message when logging in to their Heroes & Generals player account after purchasing a DLC. This bug prevents the items purchased from being added to the player account; the DLC itself is still being added to your Steam profile.
We are currently investigating the causes of this problem and are working to resolve the situation as quickly as possible.
If you are currently affected by this issue, you can assist us by doing the following:
1. Log in to your Heroes & Generals account, and when you see the error message, just type "debugwindow" (without the quotation marks).
You don't have to write it anywhere specific: just typing that word in a Flash application will run the debug mode and you will start collecting some information.
2. Once the debug window appears, click "Sv" ("Save to Clipboard") in the top-right corner.
3. Then open a text editor and paste the contents of your clipboard into it, and save the file as "XXXXXX-Flash-debug", where "XXXXXX" is your player account name.
4. Create a support ticket and add your debug info as an attachment. Then send it.
You can come back to this article later to see if the issue has been resolved.