For what it's worth, a belated reply. I was getting the same error, and looking in the web server's error log turned up the error message
Illegal field name 'APR::Table=HASH(0x7f58f28daa28)' at /usr/share/perl5/vendor_perl/SOAP/Transport/HTTP.pm line 801
A search suggested this was a bug with an older version of SOAP::Lite. Upgrading that module seems to have resolved the issue, though the RPC call response time is now monstrously long.