Hi @Ragala 18 ,
Does this issue only happen when granting permissions to specific users? Please try to grant permissions to different mailbox users within your organization to test and see if your issue would have any difference.
Also, have you ever tried to grant the same folder permission to same users using the Exchange Management Shell? If not, I would suggest you to give it a shot and see if that could help solve your issue.
one or more users cannot be added to the folder access list non local users cannot be given rights on this server
I did some research about the error message you mentioned above, most of the similar cases show that the user type might lead to similar issue. So I would suggest you to check the user type of the user you added, make sure it is a user mailbox instead of shared mailbox. For more information: Non-local users cannot be given rights to this server.
Any update, please feel free to post back.
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