Recently in our project we faced this error. Users of our system were getting this whenever they were trying to create/ update a contact.
The error seems to talk about some complex control in CRM to which the user does not have read privilege. Perplexed that you never heard of any ComplexControl entity in CRM? Don’t worry me too was.
However a little bit of search in SDK regarding this privilege and this is what I got from the SDK.
ComplexControl entity is for internal use only. However, users need read access to this entity in order to see the updated experience for lead and opportunity forms.
Still can’t find the complexcontrol entity? No worries. Open the security roles of the user who is getting this error.
Go to Customizations tab and search for the Process Configuration entity and assign the privileges for the security role. Check the below screenshot.
The SDK also clearly states out that this privilege is for Process Configuration entity.
Hope this helps!