It means the client and server disagree about the frequency PublishResponses should be arriving. The client is timing them out and then sometime later they arrive.
Between 7.8 and latest 7.9 the way timeout, number of concurrent publish requests sent, and publishing interval are all derived has changed. It wasn’t known to be buggy, I just mention this to say that 7.9 is different enough that the error message may disappear in the newer versions.