This guide described a known issue and the solution when you're using Citrix to connect to the Planner outlook add-in.
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When the user tries to use the Planner add-in through Citrix, they are met with a box asking them to login. When the user tries to login, they are met with this boxs:
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The issue happens for all users.
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The users Citrix Internet Explorers privacy settings are set as High, which prevents the use of cookies. Change the setting to Medium/High so that cookies are enabled. Then it will work again. Planner needs cookies to function.