Can you attach a screenshot of the message?
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
Then you should better contact their support.
My guess is that the server occasionally runs out of free data connection port numbers.
But anyway, WinSCP should reconnect automatically (after an interval), doesn't it?
Thanks for your report.
I have never seen this before.
What FTP server is that?
Can you attach a complete log file (as a plain text)?