Well, I cannot reproduce your problem.
What machine, OS and Fortran compiler are you using?

You can also check what your CHARMM thinks is the working directory, what files are in it, and if you can create a file there with a small CHARMM script:
* testing filenames
*
system "pwd"
system "ls -l"
open unit 21 write form name myfile.txt
stop
_________________________
Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden