Good Morning Matt
The 'DRM' is part of our DLC system. So any DLC (payware add-on) packs that you have purchased through us, or that had a serial number, will be handle by the DLC system. This system automatically downloads and installs the packs after purchase or when you first start Trainz.
In regards to error checking, this there are quite a few new things in the error checking, but it's a relatively small amount of content that has this issue these days. The main one, as you've noted, is content having errors for missing images (e.g. thumbnails).
This is only an error if you have an asset that is referencing an image/thumbnail in the config.txt, but does not have the image present. If the image is present, then the content won't be faulty. If no thumbnails container is present, then you'll get a warning. Note, if the image is present under a different name, it'll still be faulty, as the image being referenced isn't actually there.
You can fix this error by placing a correctly named image of the correct dimensions into the asset (in the same folder as the config.txt). You can find the name and dimensions in the thumbnails container of the config.txt file (note, some assets may not have the dimensions listed; for the actual thumbnail this should be 240*180 pixels).
Hotfix4 also introduces error checking on sound files, however so far I've struck maybe half a dozen assets that are actually faulty. To fix this, either replace the sound with a working one, or you may be able to import and then re-export the file using audacity which may repair the wav file.
Regards
