Can anyone explain the below "Session resumption (caching)" warning to me and how to deal with it? It's from an SSL Labs test.
This isn't a security warning at all. It is more an indicator of a misconfiguration which is bad for performance but not relevant for security: The server issues session ID's for resumption but if the client tries to resume a session using the server provided ID it will not work. For reasons this might happen see this discussion.