YES, the ER52 message repeated but that was NOT actually the problem!
I finally found a broken piece of PVA filament NOT in the material station as the ER52 error indicated, but higher, in the Bowden tube -- between the decoupler and the Printer Feeder. I disconnected the Bowden tube directly beneath the #2 feeder and there it was. Pulled out the broken piece and was back in business.
That said, here's a message I received from Ultimaker support AFTER I located and corrected the problem: "...Ultimaker has identified some cases where PVA filament breaks when loaded in the Material Station. They are actively investigating the possibilities of reducing stress on the filament in the Ultimaker S5 Pro Bundle to decrease the chance of this issue happening. In the short term, they have stated that the issue can be circumvented by cutting off one meter of the PVA filament after each print job..."
So, it seems there is likely a serious hardware problem that needs to be rectified ASAP. This solution is obviously a workaround that under the right circumstances (repeated small part building for instance) can waste an unacceptable amount of expensive filament. Imagine discarding a meter of filament each build while the amount actually used in the project is far less. You could easily waste half the spool. Not cool. For now I'm NOT cutting away any PVA filament and if the error repeats, I'll just have to locate the break.
That said, I thank you for your suggestions and help, and wish to state that aside from this hopefully temporary problem, I LOVE the S5 Bundle when coupled with Cura. And now that I know the issue is a known problem, I can for a time, work around it.
It really is a great product overall.
