Apologies, I overlooked the separate windows vs dual-display aspect when trying earlier.
(One reason we ask for one question per thread, as interwoven threads make it easier to miss things, especially when we're jumping between threads and have to re-scan them to get the context back.)
I tried again with separate windows and I don't see the upload stop, but clicking Up does fail the first time and take me to my Desktop instead of the parent folder. So something is wrong there, and maybe it's just going wrong in different ways for each of us. We will investigate that and work out what's causing it.