In short: Netbeans editor shows/highlights incorrectly, that classes and/or libraries are missing, when they aren't.
Compiling the code goes correctly, but editor shows annoyingly a lot of red wave.

I was able to reproduce this error by following way:
1) code looks fine in the editor
2) select project properties and press includes/excludes
3) includes-list consists .java files and exludes-list is left empty
4) press OK
5) problem appears and netbeans claims that some classes and libraries are missing

Fixing the problem:

1) go the "files"-tab and edit project.properties
2) remove line "excludes="
3) Netbeans editor show everything correctly and doesn't claim that classes or libraries are missing

I personally find this bug very annoying and hope that this problem is fixed. Thanks.