The answer for me was to free memory to the build process, closing programing or rebooting the machine (win10 seems to manage memory worse than win7)
Why did it fixed for me?
During the build, android studio doesn't find enough memory to compile but it doesnt show any error. The :app:preDexDebug error happened just when I tried to upload the compiled app to the phone.
Sometimes I could get through this point (no app:preDexDebug error), but when running the application on android I got "ClassNotFound" exceptions. This was the clue to discover android studio wasn't compiling all my files during build because of the lack of memory (win10 asked me to close android studio to save memory during the build), even though the IDE showing me that the build was successful.