I recently noticed Magicka got updated via Steam, and ever since I've been getting the "modding message". I tried validating my Steam cache as suggested, but the message still comes up. What do I need to do to play online.
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This is a bug that showed up in the most recent Magicka patch. The Devs are aware of it, and hoping to have it fixed by Monday, 3/21.
TL;DR: The game is working just fine, but achievements are currently not being tracked, until the next patch. Everything else is as normal.