I spoke too soon! Search worked for a few hours, maybe, but by the end of the day it no longer worked.

The first time I did an update after selecting the Beta channel, there was one update available and the results were no better. I exited Outlook, started it back up again and checked for updates. This time there were five updates available and the update process was pretty significant - had to close Outlook, Excel and Word. When I opened Outlook again, apparently it defaulted to the 'New Outlook', which wasn't supported by my installation and I was given the opportunity to switch back. I did and search is working again! For how long, I don't know. Fingers crossed.

First time I tried updating from the Beta channel, the problem was still there. I exited Outlook and looked for updates again and this time 5 updates were available. After the updates were installed, search worked again.