Hi!

When clicked on Submit for Review following error occurs under Uploaded Plugin Files:
Please remove invalid files uploaded in any of the packages.

Any ideas what could be wrong? It's a simple portlet built with maven. I've tried to remove maven files from META-INF folder but still getting this error.

Not very explicit message, isn't it?

Nope, it's not. See this thread for details. There is already a ticket filed against liferay.com for this (I too have run into this).