My guess is you have somehow managed to remove the standard IDL "library" (/user/local/excelis/idl82/lib) directory from your IDL path. This is exactly the kind of error you would expect if IDL can't find the source code for the program. You can test my theory by typing a command like this:
data = dist(200)
data = congrid(data, 400, 400)
If I'm right, you should get an error on the Congrid command, too.
The solution would be to get this library directory back on your IDL path.