Had the same thing happen.
If you use the rollover API manually to do the rollover instead of using your ILM policy, each time the policy runs it will throw this error. This happens because the index is not being flagged as complete, when the policy runs it appears to be trying to manage the index but it's not flagged as writable.
Add "index.lifecycle.indexing_complete": "true" to the index in question, then clear the error by retrying the policy. When the policy next runs(default is every 10 minutes) it should transition from the "hot" phase to whatever phase you have defined next in your policy.