maven-scala plugin not able to build scala on some projects

Hi,
One of my colleagues is having trouble compiling a scala project with
the maven scala plugin.
We are using mac osx lion 10.7.2 with maven-scala 2.15.2/maven 3.0.3
and scala 2.9.1 This project currently works on my mac.

Oddly, my colleague can compile another scala 2.9.1 project correctly.
Below is the stack trace, what can cause this type of error? It's the
first time i've seen this.

[ERROR] error: error while loading , error in opening zip
file[ERROR] error: scala.tools.nsc.MissingRequirementError: object
scala not found.[INFO] at
scala.tools.nsc.symtab.Definitions$definitions$.getModuleOrClass(Definitions.scala:655)[INFO]
at scala.tools.nsc.symtab.Definitions$definitions$.getModule(Definitions.scala:605)[INFO]
at scala.tools.nsc.symtab.Definitions$definitions$.ScalaPackage(Definitions.scala:145)[INFO]
at scala.tools.nsc.symtab.Definitions$definitions$.ScalaPackageClass(Definitions.scala:146)[INFO]
at scala.tools.nsc.symtab.Definitions$definitions$.AnyClass(Definitions.scala:176)[INFO]
at scala.tools.nsc.symtab.Definitions$definitions$.init(Definitions.scala:814)[INFO]
at scala.tools.nsc.Global$Run.(Global.scala:697)[INFO] at
scala.tools.nsc.Main$.process(Main.scala:105)[INFO] at
scala.tools.nsc.Main$.main(Main.scala:123)[INFO] at
scala.tools.nsc.Main.main(Main.scala)[INFO] at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)[INFO] at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)[INFO]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)[INFO]
at java.lang.reflect.Method.invoke(Method.java:597)[INFO] at
org_scala_tools_maven_executions.MainHelper.runMain(MainHelper.java:161)[INFO]
at org_scala_tools_maven_executions.MainWithArgsInFile.main
The pom build section:

org.scala-tools
maven-scala-plugin
2.15.2

compile
testCompile

${scala.compiler.version}
${project.build.sourceEncoding}
${project.build.sourceEncoding}

-optimise
-unchecked
-deprecation

org.apache.maven.plugins
maven-compiler-plugin
2.3.2

${project.build.sourceEncoding}
1.6
1.6
true

* please don't suggest convert to sbt :)