List index out of bounds (-1)

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.15

Error message:
List index out of bounds (-1)

Stack trace:
(00386331)
(00386331)
(0002B409)
(00278809)
(00277E50)
(003A346C)
(000436C9) USER32.dll
(000250B7) USER32.dll
(0002460A) USER32.dll.CallWindowProcW
(0047A277)
(003A346C)
(000436C9) USER32.dll
(000250B7) USER32.dll
(00024209) USER32.dll
(00023FCB) USER32.dll.DispatchMessageW

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.