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There are a couple of posts on Reddit complaining of similar issues - they all seem to have required a reinstall and restore from seed to solve.
Thanks. I discovered my phone can switch user, so I created a new account, tested restoring my seed phrase (without exposing it to a new device), and deleted my original app. Not it works again.
You could also try posting in the Coinomi subreddit - the devs are fairly active there and may be able to help.
I don't use Reddit, from what I've seen I need to score points before I can even post there, so never mind