Recently we started noticing the "error retrieving external tools" message on some assignments created at our site. I have verified the problem, and reading through similar questions, found that calling an external tool in the assignment is often a cause.
However in our case, I have observed that in several cases no LTI is being explicitly accessed as an external tool.
Is it possible that there is an LTI tool that is being called without it? Is anyone else seeing the same message?