That vague error message reminds me of a bug that should be fixed in the master version.
I can give you a newer version to test, via email. Please contact firstname.lastname@example.org
About the load balancer compatibility, there might be issues with that unless the LB supports sticky sessions.
Basically the issue that might arise is that if the load balancer simply round-robins the connections, the same browser can create its encrypted cookie in one instance and the second Kibana doesn’t know how to decrypt it.
This happens because the key for the encrypted cookie is generated in each instance randomly at startup time.