Apache Druid

The problem was solved by restarting the broker container.
How can this problem be prevented in the future?

Could there have been any old, long-running queries blocking the merge buffers? If yes, how do I check for this?

Yes it seems that is the issue.
I think you need to figure out which query is talking time and holding on to merge buffers.
One solution can be to take jstack and see which query threads are running.
They are in the form "qtpxxx" and then search for that query id in the broker logs to get retrieve the query