Whenever accessing Dynamics 365 from a bookmarked link in a new session with MCAS enabled, an error message will appear following a successful sign-in. The bookmarked link is a valid URL, but accessing from a new session will cause the sign-in to fail. As a workaround, signing in from the base environment URL listed in the Power Platform Admin Center will allow the initial sign-in to go through, after which the same bookmarked links will behave normally.

A support ticket regarding this issue was submitted on this behavior, but this was confirmed by the support representatives as an expected/by-design behavior, so I am recommending an investigation/fix for this issue as soon as possible.