Hi Ivan,
I seem to be experiencing similar problems. I tried downloading KNIME version 4.2.1, but then couldn’t access any of my workflows. Perhaps it’s related to this issue that I haven’t been able to resolve (JVM not found error when opening other workflows). I don’t want to continue uninstalling and re-installing.
So, I gave up on 4.2.1, re-installed KNIME version 4.1.4 (64-bit). I can now access my old workflows, but cannot install any extensions. I’m on a work laptop, haven’t needed to work around a firewall in the past, and even when I drag-and-drop nodes from Knime Hub, I get “could not find extensions that provide the missing nodes.”