I was having a issue with a customer that he couldn’t assign a task to other users in Dynamics 365. This issue was occurring in all locations like Lead, Account, Contact, Opportunity etc. After opening a new task on the record from for example a Lead, he was having the following error message:
After doing research on this subject, i find out that the issue occurred because the entity “Activity” in the Security Role from the user was not having the security role on “Organisation”. For this customer is chosen, that all colleagues could create tasks for each other. I set the security role to “Organisation” and published the changes and the issue was solved.