When trying to set permissions on a mailbox you might be missing the 'Save' button when using the Exchange Admin Center (EAC). In the same situation you can have the option to set and click save when setting the permissions in the Office 365 portal but you are presented with an error “Er is iets misgegaan/ Something went wrong“.
It could be that the account you are using does not have the required permissions to perform the action.
In this case the customer used a Global admin (Which was Global Admin in the Office 365 portal but wasn’t a member of the RBAC group 'Global administrators' in Azure AD)
Check if the used account has the 'Global Admin' role in the Office 365 portal - portal.office.com or else give these permissions or use a different account with Global Admin permissions
Should the information in step 1 not solve the issue then go to Azure AD and check if the account is a member of the RBAC group ‘Global Administrators'
Go to Azure Active Directory > ‘Roles and Administrators’ > Click on 'Global administrator’ and check if the account is a member, if not, then add the account
If the issue isn’t solved by using the information shown above, please contact support.