List index out of bounds (3)

<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 5.21.1

Error message:
List index out of bounds (3)

Stack trace:
(003AB465)

Status: unknown_rel_addr

The problem happens rarely. I cannot reproduce it.