A stab in the dark:
I used to use those old programming routines on the HC05s, and they were DOS based. That meant there were a number of restrictions:
No file name or directory name could be more than 8 characters.
No network names were recognized.
The total path was limited in size, but I don't remember to what.
What I use to do when I switched to Win95 was to output the .S19 file to the root directory, and access it by using "C:/filename.s19".