Hi @George Pedrick ,
Did this issue occur on a specific client only recently? If so, have you performed any specific operation before(such as install any programs), please check if there are any differences between this client and others.
According to my tests on Outlook 365 version 2105 (Build 14026.20246), no matter whether I open the mail in a new window or not, I can reply/forward the mail normally. I noticed you have reinstalled the user profile, was it a windows profile or an Outlook profile? If it's the windows profile, in order to avoid the interference of corrupted Outlook data file and profile, it's suggetsed to create a new outlook profile and readd your account to check.
If you did try it or that's invalid, based on my research, it may also be caused by any third-party programs, it's suggested to close other programs temporarily to check if the issue has any difference.
If the issue still exists, considering any errors in your register may also cause it, please try to rename your Outlook register key(Location: Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook, rename it to Outlook1), then restart your Outlook and readd your account for testing.(Note: Serious problems can occur if you modify the registry incorrectly. Before making changes, back up the registry to restore it in case something goes wrong.)
If your issue has any update, please feel free to post back.
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