Hi Harry,
That error generally boils down to subscribers consuming from topics slower than publishers publishing to them, creating a bottleneck. This is covered in a few places, but some documentation and discussions are below:
https://www.hivemq.com/docs/hivemq/4.11/control-center/analytic.html#dropped-messages
https://community.hivemq.com/t/qos-0-memory-limit/115
It may seem like a good idea at first to try to increase memory allowance when encountering this error, but in the end it is often a trap - increasing the memory used by the in memory queue without addressing the client consumption problem may only end up giving a higher ceiling before the problem reoccurs.
Thanks,
Seth - HiveMQ Support