I've gotten that quite a few times when /devmap'ing, even with etmain/etpro 2.0.7 - so it's not an ET Pro 3 thing.
I find just restarting ET and trying again normally does the trick. And if not - clean out/move a few pk3's in your etmain dir.
( if you have > 100 MB or just lots of pk3's in the etmain dir of a server, it crashes - and /devmap makes your install the server..
![Neutral :|](./images/smilies/icon_neutral.gif)
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