Hi Joost,
It does appear to be the new Google Drive filestream client, specifically the caching mechanism for "offline files" that stores the files ~/Library/Application Support/Google/DriveFS while representing them to the OS under /Volumes/GoogleDrive/
The error will occur if the individual images haven't been loaded up/viewed in another app beforehand (Photoshop, Preview). This is consistent with incredibly slow file copies etc in and out of "locally stored" directories as well. I simply think their caching is really suspect on the Mac.
If I've looked at each image individually in another app before trying to stitch, PTGui behaves fine, same as having files stored on the desktop.
So... resolved, I guess :)
Best,
Trey