Microsoft initially said that Office 2011 would not work with High Sierra, but a fully updated Office 2016 would - one might imply that included Outlook.
I had Office 2016 / 365 installed on my mid-2011 MBA, as part of a work thing (though I hardly used Outlook, as one of my email accounts was Gmail, which Outlook still doesn't like) - and I didn't see issues. So yeah, it could just be a reinstall issue, now that its dealing with the new file system.
And, as an interesting sidebar, when I uninstalled Office 2016 / 365 (work change and I hate the idea of a 'subscribed' Office Suite product), I took a chance - given what Microsoft had said - and reinstalled my own Office 2011 on High Sierra. No issues at all (albeit not using Outlook here either), so tempted to say the installation of High Sierra was used an excuse to force users into newer / subscriber product - especially as 'support' for Office 2011 runs out October 10.