Check your file permissions in the /out dir and delete the zip file there. Had the same problem a few months ago and that worked for me.
Hmm.. did you try uninstalling Eclipse and re-installing it? Maybe something has gone corrupt.
"Roku SDK DOC wrote:
The new plugin required Eclipse 4.5 (Mars) as a minimum, DLTK 5, and Java 8.
So I set out to try and duplicate the version that was probably on the developer's workstation at the time the new plugin was announced on the Roku Blog April 20, 2016. That would have been Eclipse Mars 2 (4.5.2) and DLTK 5.4.