Problem fixed: build.properties in %HOME%/.netbeans/6.7rc3/build.properties contained a lot of references to an old
installation.

libs.CopyLibs.classpath=[...]\\NetBeans 6.7
nightly\\java2\\ant\\extra\\org-netbeans-modules-java-j2seproject-copylibstask.jar

After replacing all occurrences of "nightly" with "RC3" everything is fine:

libs.CopyLibs.classpath=[...]\\NetBeans 6.7 RC3\\java2\\ant\\extra\\org-netbeans-modules-java-j2seproject-copylibstask.jar

I exported all options from the nightly build installation befor installing RC3 and imported them in RC3 (using the new
function in den Options dialog). 

I guess that netbeans jars should have a relative path (or variable) in build.properties so that export/import works.

I'm seeing this same issue with final on OS X. 

I looked at the build.properties and have no references to prior installs (no RC1, RC2 or RC3 anywhere in the build.properties file).

I just ran into this issue; many thanks to lallafa for pointing me in the right direction. Specifically my "6.7" install
was referencing a "6.7 RC2" install, and this afternoon I was doing some clean up and removed the RC2 userdir (the
uninstall has that convenient option (new isn't it?) to remove the userdir associated with the release being removed)
and I used it. Essentially I had to do ":%s/ RC2//gp" to build.properties and all seems ok now

This may be a bug, or an unfortunate interaction, with the new IMPORT/EXPORT OPTIONS feature. When I first started up
6.7 I declined to have it import anything during install. Later I did an IMPORT and navigated to the '6.7 RC2' userdir
to import from there. I can't think of any other opportunity for the pollution.

Since I'm not certain of what happened or other possible ramifications, I'm bumping the priority; it seems like there
could be nightmarish problems going forward.

*** This issue has been marked as a duplicate of 166585 ***

I'm seeing this in 6.8 RC1.

In my case the RC1 has its own user folder so the explanations here doesn't apply to the issue I'm seeing or the duplicate issue that was closed either.

Here's the compilation error:

Z:\Netbeans\Xinco\xinco-2.01.00\XincoWebApp\nbproject\build-impl.xml:397: The following error occurred while executing this line:
Z:\Netbeans\Xinco\xinco-2.01.00\XincoExplorer\nbproject\build-impl.xml:484: taskdef class org.netbeans.modules.java.j2seproject.copylibstask.CopyLibs cannot be found

And the related line in build-impl.xml:
<taskdef classname="org.netbeans.modules.java.j2seproject.copylibstask.CopyLibs" classpath="${libs.CopyLibs.classpath}" name="copylibs"/>

The issue only happens on Web projects. It works well on Java projects.

Updated affected version to 6.8.

This might be the same as Bug 177158 which is related to Options>Import issues as well.

That bug fix just missed the 6.8RC1 but is available in today's dev build.

Yes, it should be fixed.

*** This bug has been marked as a duplicate of bug 177158 ***