It's been showing that, and not letting me do anything in CC, for almost 24 hours now. If I restart the phone, it goes away, but comes back as soon as I go back into CC - I've done that twice, once after about 30 minutes and once about an hour ago.
How I got here: many of these may be irrelevant, but I'm putting in everything I can remember.
Samsung Note 3, Kitkat
Calibre on PC - was 2.9, upgraded to 2.13, on two different computers sharing a database via Dropbox.
Successfully synced many times on this phone, using wireless connection and occasionally content server.
I'd added some books on one computer. Went to the other computer, added another book, realized the new books weren't on the second computer, closed down calibre and reopened it, had to re-add the book I'd just added (conflicted metadata.db, I deleted it).
Then tried to connect CC, to get that latest book onto my phone. Opened CC, connected via wireless, it stalled out "waiting for calibre". Closed CC, reopened, connected again; it was very slow, but did connect. Finally started downloading metadata; stalled at 6%, calibre said "error connecting to device". Closed CC again, reopened, got a blank page with the CC header and a notification in the top bar, a red book that says "Database Upgrade: Book 0 of 1". Let it run for about 30 minutes-1 hour, restarted my phone. Opened CC, same thing. Let it run overnight, no change. Restarted the phone again; when I opened CC, it said "Grouping disabled" at the top (and "Slide for grouping drawer" below that, but otherwise the same blank page - notification reappeared, has been running about 2 hours now. And I can't back out of CC, have to use the home button.
I don't think I have a new version of CC - I don't remember when it updated last, but it's not in the Recently Updated section of the Play Store. I did update calibre recently, but the stuck database is on the phone...
Should I uninstall CC and reinstall? I'm using CC's folder, so that will wipe my books - not terrible, though I may lose a couple read dates (I keep all my books on the PCs, I'd just have to resync them).
I tried a sync to my tablet - Nexus 7 2012 running Lollipop 5.0.0. Seems to be working fine. Yep, metadata transferred both ways, downloaded a bunch of books, no database update (or not one that lasted long enough for me to see).