Just confirming, we encountered this exact problem in production (unfortunately the error did not pop up for us as part of the Moodle upgrade that got us into this state). The steps above resolved the problem for us as well.
I am curious to know why a particular 7.4.x version of PHP combined with Moodle would result in XML issues talking to Turnitin. Does it cause incorrect XML to be generated? Perhaps it causes correct XML to be generated, but in a form that Turnitin doesn't like?