@olivierlambert Beyond 'Did you try rebooting it?', I think 'Have you updated to the latest version?' is the second hard and fast rule all IT people should follow. I suspected the problem was that I was a few releases behind, so I ran an update. It brought the software to 5.109, though I didn't check the blog release page to confirm if I was in fact on the latest. I can finally sleep well tonight!

@olivierlambert Updating to Xen Orchestra 5.110 fixed the issue