iCloud for Outlook causes Outlook to hang

I have several machines that are all running Outlook 365 for business. Most of them work fine with iCloud for Outlook, but one refuses to start if iCloud integration is turned on. Once it starts, clicking anywhere in Outlook causes a "not responding" message and a grayed-out window. If you close Outlook and try to reopen it, you get a message that the xxx.ost file cannot be opened (one for each account) and it won't do anything until you restart the computer.

I have tried reinstalling iCloud (now from the MS Store), repairing/reinstalling Office, OS file checks, etc. and have not been able to get it working. The strange thing is that the same configuration of accounts, iCloud, etc. works fine on other machines. It's just this one.

The iCloud add-in is present in Outlook, but there is no menu item on the ribbon and no data file for iCloud, and that works fine but the iCloud calendar is not present. Once I sign out from iCloud and re-sign in, then enable integration, the data file is present and it hangs Outlook.

Any ideas of what to try next?