I have tried and this does work.

- Define a project
- include a file1.txt file with

file Message

text.

- a Main.prg with

SYS(2450,1)
MESSAGEBOX(FILETOSTR("file1.txt"))
h=FOPEN("file1.txt")
MESSAGEBOX(m.h)
FCLOSE(m.h)

Compile and run. All OK:
FILETOSTR() is ok
and
FOPEN() !!! is ok