I wanted to report the exact same issue that chrj reported, with exactly the same behaviour when I ran PLSQL developer with the DebugSQL paramter.

I have just upgraded from version 8 to 9.04, then I tried it with 9.05, hoping the newest version would resolve the error.

I was receiving both the "Invalid Filename" error and the Access violation at address..." error. Sometimes one, sometimes the other, never both at one time. It seemed to change when I closed and reopened the application, but remained consistent until it was closed again.

I ran it with the DebugSQL parameter and now it works just fine, even when I run it without.

I can't reproduce the error with DebugSQL turned on.