Thanks for your report.

This issue has been added to the tracker:
https://winscp.net/tracker/1764

If you send me an email, I will send you back a development version of WinSCP for testing. Please include link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

The issue is the multipart chunk size...
This is defined in the libs3. (s3.c)
#define MULTIPART_CHUNK_SIZE (15 << 20) // multipart is 15M
Making this 30M would allow for 100GB transfers, 60M for 200GB transfers.

The upstream project (libs3) has updated this to:
#define MULTIPART_CHUNK_SIZE (768 << 20) // multipart is 768M

Can winscp please be rebased with the updated libs3?

sorry, this occurs at 50Gb, not 100Gb.

Error: Part number must be an integer between 1 and 10000, inclusive Extra Details: ArgumentName: part Number, Argument Value: 10001,

Winscp should use a larger value for the multipart, or allow a configuration option to set this.