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Fix for “Recent changes couldn’t be saved” in Project for the Web / Planner (Schedule Tab)

I finally found the root cause of this issue, and I’m posting it publicly to save others a lot of time.

When you make updates in the Schedule tab in Project for the Web / Planner, the system does NOT run the action under your own user account.
Instead, it executes the updates using the Project/Plan Owner’s security roles and privileges.

If the Project/Plan Owner is missing required permissions, the update fails — even if you are a System Administrator.

To fix it, make sure the Project/Plan Owner has at least Read/Write privileges on these Dataverse tables:

msdyn_psserrorlog (PSS Error Log)

msdyn_resourceassignment (Resource Assignment)

msdyn_projectbucket (Project Bucket)

Once those privileges were added, the Schedule tab updates started working immediately.

Hopefully this helps someone else avoid the same debugging nightmare!