The issue has been resolved already. The fix will be included into the next release.

code65536 wrote:

In the new 4.0 beta, if I create a session without a username (so that I am prompted for one during the connection process) and then I try to transfer a file from the server in the background, WinSCP will pop up a prompt asking me to enter my username.

This does not happen if I create a session with a username.

(Also, the prompt that is popped up during the transfer will not go away even after I have entered the username; it will remain there, modal, and I am forced to close WinSCP through the task manager)

Hi, just to inform you and Martin: I got the same experience.
It doesn't repro with the 3.8.2 release.

Thanks for report.

In the new 4.0 beta, if I create a session without a username (so that I am prompted for one during the connection process) and then I try to transfer a file from the server in the background, WinSCP will pop up a prompt asking me to enter my username.

This does not happen if I create a session with a username.

(Also, the prompt that is popped up during the transfer will not go away even after I have entered the username; it will remain there, modal, and I am forced to close WinSCP through the task manager)