Beta 11: eM Client has stopped working

I’m getting this Windows dialogue box popping up quite frequently, meanwhile eM Client Beta 11 seems to be working quite happily despite it. It happens seemingly randomly, whether I am in the program or doing something else.
I’m guessing some diagnostics would be useful - what would be most useful?

Following up, there are seven eM Client processes running, and clicking “Close the program” on the dialogue box stops just one of them. Not sure how significant it is, but it is always the fifth in Task Manager, which is immediately above the one titled “Inbox …”. All the others are just labelled “eM Client”
I found if I just minimize the dialogue window it doesn’t seem to pop up again.