SIP Login fails with 21.7.5

[chefhb88]

After updating to 21.7.5 there is a problem with SIP login to 1und1.
My tech stack is a Fritzbox behind OPNsense. NAT Rule is active as explained in other topic.
Now after i would say years of no problems my Fritzbox brings up error message about connecting to
the sip provider "Error 403".

I've double checked every setting in Fritzbox, registered a new phone number to test about the user/ password.
Checked OPNsense config. Disabled IPS and so on. No solution.

Now after reverting back to 21.7.4 problem is gone...