I've run into an interesting problem where S-XML-RPC would complain that an "empty" string was not of type LIST. It would only occur when I used S-XML-RPC for just the server. When I used something else as the client, like the command line tool that came with the XML-RPC-C library or the XML-RPC.Net library for C#, the server would return the fault. Using S-XML-RPC as a client worked fine.
After some debugging, I think I traced the cause to a few extra #\Newline and #\Return characters getting inserted into the XML from these other libraries/tools. So essentially, they give us a string that should be empty, but instead has whitespace in it. I'm not sure if it's more correct to patch these other libraries instead, but the small patch I'm attaching fixes this in S-XML-RPC.
Cheers, -Andrew Jones-Gonzales