Very strong echo, normal explanations don't seem to apply

A couple of weeks ago I had a small meeting with 5 people and there was terrible echo, as if in a large reflecting hall. We meet regularly and in the past  we’ve resolved issues with eg logged in on 2 devices, logged in via browser and app simultaneously. But this was worse than any of those. 

In my settings I’ve got original sound for musicians On, and Echo Cancellation On. Advanced > Echo cancellation is set to Auto

Timeline: I opened the meeting and person A joined. Immediately severe echo.  So looks like problem is me or person A. After the others joined we did the usual routines of muting each person at a time, then tried muting everyone and un-muting one at a time. No difference, including muting A made no difference. So I closed the meeting, rebooted, and started the meeting again. This time person A joined last, but we still had the echo reight from the start of the first person joining. (Though the quality of the echo was a bit different.)

So this time the problem couldn’t be A because he joined last, and couldn’t be me because I’d just re-booted.

Does anyone know what’s going on? Am I missing something? If it happens again, are there different tests I should try?

Many thanks