Re: "Ignoring roster push with not exaclty one entry." - Is this right?

Smack is right.

See RFC 6121 § 2.1.6 Roster Push:

The following rules apply to roster pushes:

 

1. The <query/> element in a roster push MUST contain one and only one <item/> element.

I agree RFC 3921 is not very clear about it, but it's obsolete long enough, that Tigase could have updated their implementation.