Latest Update: Jan/7/2020 => This issue is fully resolved for IE11 after recent service update. You can re-enable Profile Cards by turning ON Org wide setting - "Enable users to view contact cards"
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Past comment Dec/7/2019 =>
There is a temporary issue with using Unified Client interface and Profile Cards feature in Microsoft Internet Explorer 11 where opening a case or contact record and hovering on Lookup displays a blank page.
This Issue is getting fixed soon but as a workaround, users can access Profile Cards from an alternate Internet browser or open the contact record to get the desired contact information.
Please see if this is due to People Cards related issue, if so you can temporarily disable Profile Cards by turning off Org wide setting - "Enable users to view contact cards"
More information about Profile Cards can be found on the Docs page-
docs.microsoft.com/.../enable-profile-card
docs.microsoft.com/.../profile-cards