I run my server on a slug too (debian slug) but with version svn-1376. I got the same message when trying to upgrade to svn-1696 (so I got back to svn-1376).
I saw a similar post on the forum but it didn’t get any positive answer: http://forums.fireflymediaserver.org/viewtopic.php?p=16062
I saw other similar posts too around FirePlay
What is strange is that it works fine on FireflyMediaServer on Windows. (svn-1696)
It may be linked to the arm version for the slug.
FireflyClient is based on the xml version of the protocol. It crashes too when requesting the songlist from a web browser. I don’t understand why.
I will try to implement a rsp based version. Indeed, RSP requests for songs list seems to work well.