HSERIN/OUT require you to manually manage over-run errors and suchlike in the USART. This is hinted at in the paper manual but not mentioned at all in the on-line help that comes up in MicroCode Studio.

Unless you manually fiddle with the USART registers you will have a very unreliable Tx/Rx system. I spent days farting around with this before I dumped HSERxx and went back to SEROUTx that works every time on any pin.