I found this answer, 2 pages deep on a Microsoft community forum.

Worked like a charm. I don’t get a lot of calls for this, so I will assume it’s a random issue.
Go to File -> Options, then under Outlook Options window select Advanced and scroll down to under Other section, disable the box for the : Use animations when expanding conversations and groups and do a restart of the Outlook again to verify if this resolves the issue.
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