Andrey Fortunately the problem does not occur today! Looks like rebooting the computer helped. I ordered an external HD which I will receive tomorrow and try to place the Main archive on it. We will see whether the problem will return.
An addition: I was too optimistic, the problem haven’t vanished completely. I managed to change “Time per slide”, but"Destination" in “Import options” quite often shows only “Pictures”. I can live with it and I hope that in time I will find some logic governing this behaviour.
P.S. I experimented with several photos management programs including QuMagie on QNAP NAS and I found your program best suiting my needs. Thank you!