Migration to Lightroom CC stuck?

Hi,

I'm in the midst of a Lightroom CC migration, but I'm worried it's frozen. I left it over night, but saw very little progress. In short, it seemed to have stopped, except for some additional photos were added when I first checked in the morning. Since then, however, no progress, suggesting it did some work after I last checked at night, but then stopped. Out of my roughly 110,000 image library, Lightroom CC shows 57,000 migrated. It's stuck at that level for hours now, with the CPU working quite hard (dynamiclinkmediaserver is the active process). Meanwhile, when I log in to the mobile app or web it shows 4100 photos as in the library.  This also has stopped increasing. The web app identifies that other photos as "sync issues."

At this point is there a best course of action? Should I quit the migration? Might it start working again? Technically the app is responsive, as I can go into preferences and make changes.