Hi there!
Just an info: If I got it right this started after updating KNIME (topic: Download new nodes) from 3.4.1 to 3.7.1 (I guess you went for a newest version). First thing I would try is to download and install a fresh installation of newest KNIME version, transfer workflow there and see does that help. This is under assumption you did update through KNIME GUI.
Br,
Ivan