I am by no means an expert, but since no one else has responded, I'll share my experience with you.
I use VISA to communicate serially in LabView. I've seen lots of VI_ERROR_TMOs that appear to be normal time-out events on the port. When I've seen them, I am reading from the port, and this tells me that there is no data to be read.
When you are writing to the port, Perhaps it means that the port is still busy with a previous command, or busy receiving. Does this port have a write buffer? Try increasing the buffer size.
I hope this is helpful.
-Jim