USBOpen(), USBSend() and USBRecv()

[gehakte_bits]

Not familiar with your terminal program. But you could monitor the Async link data to see what side loses it.

It sounds more like a buffer overflow somewhere (sending data too fast)
But using a terminal program like YAT, one can set the output buffer to 60 bytes, buffer no more than baud rate, buffer chunks of 60 bytes, and sending max bytes / time under advanced settings and other things to play with.
Xon/Xoff also might help (I think that is on by default on the FT260 if I remember) so turn that on on your terminal program.