It seems that IDEA 10.5 EAP is not compatible with the current (nightly) version of the Scala plugin (#0.4.732).
As IDEA EAPs are quite stable in general, there is a fair amout of developers using them for daily work (because they typically contain useful new features).
At the moment, I'm getting an AbstractMethodError exception often: http://youtrack.jetbrains.net/issue/SCL-2952
Since Pavel mentioned that #0.4.732 is a candidate for a stable release, you might want to set the until-build, as I imagine I'm not the only one using IDEA 10.5 EAP.
If the API compatibility difference is small, it would be even better if the plugin worked on both 10.0.x, as well as 10.5.x ;)