Around line 2283 in swank.lisp is a function, swank-require. It calls CL's `require' passing three arguments.
Now, the third argument is determined by a call to `module-filename', just a few lines down, which triggers an error if it can't find the file.
The reason this is a problem is that I bind swank into my SBCL core and then I move that core to other systems, which don't have the swank Lisp files on them. Although a regular call to require works fine in this case, swank generates an error because the .lisp file itself cannot be found.
I think the solution is for module-filename to just return nil if it can't find anything, and leave it up to `require' to generate the error if need be.
John