This error can be encountered when making a Boomi Marketo Rest Connection, either when attempting such a connection, or when attempting to import a Profile. There are two common causes of this issue, which can be identified by the second half of the error message.
1. "Error communicating with Marketo service:  Only HTTP or HTTPS resource URIs are allowed here"
This error indicates a problem with the URL set in the Marketo Rest Connection. Check and make sure the URL is correct, and that there are no leading spaces (or other whitespace characters) at the beginning of the URL.
2. "Error communicating with Marketo service: [-1] null"
This error indicates that you are attempting to pull too many fields in the Marketo Rest Operation. The exact number of fields that will trigger the error varies, but testing shows it to be between 300 and 400, depending on field sizes and details. Select fewer fields and test again, until the connection begins succeeding.