I got this error when configure SSL VPN in NGFW 5.8.1
2015-03-11 15:47:39.112 sslvpnrwpd.conf info: Loading SSL node
2015-03-11 15:47:39.164 sslvpnrwpd.conf info: New config parsed: result=ok
2015-03-11 15:47:39.446 sslvpnrwpd.net error: bind() returned [-1], errno (Address already in use), Port 
2015-03-11 15:47:39.447 sslvpnrwpd.conn error: bind error on [10.0.2.52]:555
2015-03-11 15:47:39.447 sslvpnrwpd.default error: Failed to start Listener
2015-03-11 15:47:39.447 sslvpnrwpd.conf info: SSL Credentials  not found in current configuration
2015-03-11 15:47:39.447 sslvpnrwpd.ssl info: No ssl credential configuration found for ssl credential id 
Does anyone have any ideas?
Thanks and Regards!
the "address already in use" indicates you already have something listening on port 555. It's a nonstandard port so if something is listening, it must have been configured by admin (like browser-based authentication).
The credentials not found is probably more dire though, do you have server credentials selected in the SSL VPN Portal element in SMC?