When searching in the Regarding lookup field of Dynamics 365 App for Outlook, no records are returned. You see a message that says "No records found. Create a new record."
Microsoft is investigating the cause of this issue but this is typically caused by something cached on the client computer.
First try clearing the browser cache in Internet Explorer:
Note: Although you may have other browsers installed on your computer, Outlook desktop always uses Internet Explorer when rendering web content such as inside Dynamics 365 App for Outlook.
Clear the cache in Internet Explorer.
Close all Internet Explorer and Outlook Desktop windows and make sure through Task Manager there are no remaining Internet Explorer or Outlook processes running.
If this issue only appears in Outlook desktop and clearing the cache via Internet Options does not help, try clearing it via F12 chooser (not applicable for Windows 7):
Open Dynamics app in Outlook desktop
Open IEChooser.exe in %WindowsFolder%\System32\F12 (example: C:\Windows\System32\F12)
Click Dynamics 365
Click the Network tab and then click the Clear cache button.
After following the steps above, reopen Outlook and the Dynamics 365 App for Outlook to see if the issue is resolved.