ADTPro 2.1.0 client crashing to text page 2 only with specific target modern PC

I'm running ADTPro 2.1.0 from a 5.25" diskette on a IIGS. The disk has been sector verified, the drives have been thoroughly tested. 

When connecting the IIGS to the native serial port on one of my windows XP machines (either a dual pentium pro 233MHz or dual pentium 3 850MHz) everything works perfectly. The pentium pro machine doesn't seem to keep up full transfer speed (the CPU overhead of the server applet must be extremely high) but the P3 box handles it fine. When connected to either of these server machines I can send and receive images of 5.25" disks, 3.5" disks, or even 32MB partitions on my microdrive turbo with no errors or problems of any kind. I'm using a 3rd party premade "printer" cable from ebay. 

However when connecting to my modern PC using the exact same hardware, transfers of larger images (3.5" or CF card) will just about always crash out after some number of seconds. Sometimes it runs longer, sometimes shorter but it always fails. When it does fail, the ATDPro client crashes to text page 2 (screen shows nothing but '2' in every character cell). If the transfer was from apple->PC the server program will start creating a bunch of files on the modern PC with gibberish names. If the transfer was from PC->apple I get a corrupted disk. The modern PC that causes the issue is a Ryzen 3700X running in a biostar MB with a built-in serial port on the motherboard. Windows 10, NVMe SSD.

I've ordered a USB to serial dongle to see if the problem is specific to the built-in port on the biostar board, but it's not here yet. However, I would say that a client crash being triggered by serial communication errors is certainly a bug.