check that server supports urn:xmpp:ping before sending
It would appear that Gajim blindly sends urn:xmpp:ping messages to the jabber server without first checking that the server supports the feature (or ignoring that it doesn't).
\<iq to="server" type="get" id="405" from="user@server/resource">
\<ping xmlns="urn:xmpp:ping" />
\</iq>
\<iq to='user@server/resource' type='error' id='405' from='server'>
\<ping xmlns='urn:xmpp:ping'/>
\<error code='404'>Not Found\</error>
\</iq>