Symptoms

End user is unable to access a shared calendar they have added to Outlook or they can access it but they can't create any appointments on it, despite the permissions being correct. This is all whilst the Mail Manager add in is running. If the Mail Manager add in is disabled, they can access the shared calendar and create appointments on it just fine.

They may also find that if they right click on the shared calendar and bring up the properties it doesn't show the permissions, possibly even when Mail Manager is disabled.

This has also been reported as causing the PC to hang and/or crash when sending emails too and was fixed by applying this change. (Note that another separate cause of this specific symptom is also documented here: Resolving graphics driver errors/Outlook crashing when sending messages)

Fix

Go to File > Account Settings > Account Settings to bring up the email accounts

Double click on the email account to edit it and then click More Settings.

On the Advanced tab disable "Turn on shared calendar improvements"

You will need to restart Outlook for that change to take.

Accessing a shared calendar will then work just fine.