Well it doesn't have to be a script, even putting "//write -l1 somefile.txt this is a test" into the command line, it will write it first, create the file, but then I get the "* /write: unable to open file" error if I try again. (and the file isn't write protected).
"/write somefile.txt this is another test" works fine, but whenever I use parameters it gives the error.
But the best thing is, it doesn't happen when you first run mIRC, it works fine, let it run for a day or 2, writing away, then the errors start appearing. My mIRC is currently up 2 days 15 hours, and i am repeatidly getting the errors.
Maybe try letting a timer run for a cpl of days and then you will see the errors.
/timer 0 60 /write -il1 somefile.txt this is a test
Russ