If you're attempting to build an LTI tool provider that supports Deep Linking aka Content Item Messaging and you see the following symptom described in Behind the Blackboard!
"LTI TP posts back to /webapps/blackboard/controller/lti/contentitem fails. When posting back to /webapps/blackboard/controller/lti/contentitem Learn fails with "The specified resource was not found, or you do not have permission to access it."
You can fix the issue by addressing two items in your code:
a) Blackboard Learn sends an optional "data" field with the LTI launch. Be certain you pass it back to Blackboard Learn.
b) The ContentItemSelection request sent to BB must be signed.
The Partner that originally reported the issue got back to us and said they did the following above two items to their Tool Provider code to resolve it. In the meantime our automated processes created a known-issue article, then when the bug was closed by our LTI engineer as unable to reproduce it was set as being resolved in 3400.1.0. It's not a bug and functions as designed in all versions of Learn that content-item messaging is available.