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FIELD_CUSTOM_VALIDATION_EXCEPTION is thrown by field validation rules in SFDC. It isn't possible to see what caused this from the Marketo side. I would start by looking for rules on Campaign members. In SFDC go to Setup and search for Validation in the sidebar. There should be a menu under Campaign Members. If you can't find the relevant rule there, then you'll want to look under the menu for Campaigns, Leads and Contacts as well.
Thank you so much for the insight! Sure enough, there is a rule in SFDC restricting campaign member access for the Marketo profile. I'm working with the SFDC Admin to get this rule deactivated.
We used to get that error quite a bit due to the visibility rules we had set up for the Marketo integration.