Bad VOIP Quality since Update to 9.502-4

Hello,

since updating the UTM to 9.502-4 the voip voice quality (rtp) is pretty bad.
Sometimes crackles very annoying (than it's hard to understand the other one) and sometimes everything is fine.
And sometimes it changes from good to bad while the calles.

Before the update we never have had such problems. All SIP/RTP are routed via multipath rules over a dedicated uplink.
This uplink has more than enough bandwith to handle the calls and the switch shows 0 packet drops.

Any hints?

Thanks in advance,
Rafael.