Getting errors 603 and 1018 with CRM integration active

Hello, I am using Marketo with a CRM integration and the following permissions for an API user:

-Read-Write Activity
-Read-Write Activity Metadata
-Read-Write Assets
-Read-Write Campaign
-Read-Write Lead

I am getting both 603 and 1018 errors. After running a test with the CRM integration disabled I still got the 603 error but after adding the Read-only Company permission along with the above the errors disappeared.

We need the CRM integration to remain active on the live account but it is my understanding that enabling the Read-Only Company permission while having this integration active would still return 1018 error, therefore what other causes could there be to these errors?