Try the following:
1. Remove your username and password from the internet tab under Ttrainz settings until it fails, then re-enter the login info until it activates, or;
2. If you know the DLC folder of that DLC, delete all of the folders for that DLC (ie sc450_1, sc450g, etc), then issue a database update, and then file > download purchased items;
3. If those 2 fails, contact the helpdesk
Cheers