The error is this typical, pesky “an internal build error has occurred. right-click for more information.” error. For “normal” causes of this error see:

• 9mmedia blog post 165
• Juda Frangipane blog post 212
• Michael Imhoff “Internal build error” blog post
• more searching through the Internet

Now all this did not work in this case. The {$WORKSPACE_LOC}/.metadata/.log said:

!ENTRY com.adobe.flexbuilder.project 4 43 2009-12-30 22:49:43.166
!MESSAGE ConfigurationProblem logging info
!STACK 0
java.lang.IllegalArgumentException: "The attribute value type is com.adobe.flexbuilder.project.compiler.internal.ProblemManager and expected is one of java.lang.String, Boolean, Integer"
        at org.eclipse.core.internal.resources.MarkerInfo.checkValidAttribute(MarkerInfo.java:84)

In Danyul’s post 68 I found the solution: when trying to combine Flex Builder for Linux 3 Alpha 5 with Eclipse 3.5 (which is officially not possible), the compiler will halt on all warnings! So cure the warnings in the project that throws the above error, and it will compile.

The cleaner solution is, of course, a patch that sorts out the ProblemManager problem. Installing this is discussed in above-mentioned Danyul’s post 68. There is even an unofficial port of Flash Builder 4 beta to Linux (referred to here and in various other blogs), but I could not get hold of the files.