If you have a REST API integration with Sugar, you are likely using a custom platform as part of that integration. Beginning in the Winter ‘18 release, Sugar is enforcing a rule that all custom platforms must be registered in each Sugar instance where a custom platform is used. For more details on the rule itself, see Matt’s earlier blog post: Unknown platforms to be restricted in Winter ‘18 release.
The bad news? This change that requires all custom platforms to be registered probably affects you.
The good news? There is an easy fix, and I’ve written a tutorial that walks you step-by-step through how to implement the fix.
Sugar Developer Blog - SugarCRM 