umm, just out of curiosity: why would that change anything if the driver only connects to a single node replica set?
However, I can unfortunately not try that easily, because I haven’t seen that exception ever since. Tried to replicate, doing the same things, but it never showed up again so far.
Our web clients (browser) connect to a web socket, which modifies the current user. There is a chance that this operation took place when a transaction (which also modifies the user document) was still running. Just a vague guess, but I haven’t found anything how I could check if this really is what has happened.