I started the upgrade of the router via the Omada web interface (via my OC200) on Tuesday or Wednesday. At first it showed no progress (I was able to click the upgrade icon again), today it shows this:

(it isn't clear on the screenshot but the upgrade icon is shaded and currently isn't clickable)

FYI: the router rebooted via a schedule to resolve gradual increase in resource usage over time

Any ideas on how I can force the upgrade to continue or restart? 

Update: I noticed a possibly related thread under the standalone router section: https://community.tp-link.com/en/business/forum/topic/267282?replyId=604608

However, that still leaves the question of what state my router is in unanswered.