I have a site workflow that is triggered by a list workflow in the same site (Start Workflow action), and a list workflow in a different site collection (web service call). The workflow runs to completion and does everything it's supposed to do when triggered from either source. When triggered by the Start Workflow action, everything is perfectly normal. When triggered by the web service call, the workflow throws an error, and all of my customer workflow logging notifcations are missing.
When triggered with the Start Workflow action, normal logging messages appear:

When triggered with the web service call, the workflow runs successfully to completion then fails with this output. None of my custom logging is present.

The error log has a message that the workflow could not update the item; however, multiple items are updated and it updates everything it's supposed to.
Any ideas?
