Unlike any other game + expansion in Steam's catalogue (that I'm aware of), AP and CW share the same ..\steamapps folder structure. Verifying the integrity of game cache files (GCF) is a sure way to break both games at the moment.
If anyone wants to play just Crossworlds and do not ever intend to run AP again, you can fix any issue as follows:
Install AP, verify GCF.
Install Crossworlds, and (to be certain) verify GCF.
Play only Crossworlds from this point and you should be fine.
If you launch AP and/or have received an auto-update from Steam for AP, crossworld will be broken again.
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