Cannot register new T42s anymore

[roli8200]

Tried to register new T42s (Firmware: 66.84.0.15) against FreePBX (made this more than 100 times successfully before). Its a brand new one, right out of the box.

The new ones seems to ignore the response Status-Line: SIP/2.0 401 Unauthorized
and do not reregister with the credentials included.
It tries to register again and again but never sending credentials to the server.

It sends always this registration (replaced name with XXX), but always without credentials.
Via: SIP/2.0/UDP 10.250.250.10:5060;branch=z9hG4bK3423351914
From: "XXX XXX" <sip:510@192.168.16.25:5160>;tag=79229429
SIP Display info: "XXX XXX"
SIP from address: sip:510@192.168.16.25:5160
SIP from tag: 79229429
To: "XXX XXXXX" <sip:510@192.168.16.25:5160>
SIP Display info: "XXX XXX"
SIP to address: sip:510@192.168.16.25:5160
Call-ID: 0_3734511584@10.250.250.10
[Generated Call-ID: 0_3734511584@10.250.250.10]
CSeq: 1 REGISTER
Contact: <sip:510@10.250.250.10:5060>
Contact URI: sip:510@10.250.250.10:5060
Contact URI User Part: 510
Contact URI Host Part: 10.250.250.10
Contact URI Host Port: 5060
Allow: INVITE, INFO, PRACK, ACK, BYE, CANCEL, OPTIONS, NOTIFY, REGISTER, SUBSCRIBE, REFER, PUBLISH, UPDATE, MESSAGE
Max-Forwards: 70
User-Agent: Yealink SIP-T42S 66.84.0.15
Expires: 3600
Allow-Events: talk,hold,conference,refer,check-sync
Content-Length: 0

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Update: Downgraded firmware to 66.81.0.70. Works flawless with this firmware version.
Cannot make tests with all firmware versions, but 66.81 works.

Is there something wrong with 66.84?