Lucas Miller from the GP team explains an eConnect Error 11991 related to GP 2013 R2.
eConnect Error 11991 – Microsoft Dynamics GP Community
Install, Upgrade & AdminDevelopment & IntegrationMicrosoft & the GP EcosystemGP 2013
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.