AD7091R-5 using internal reference


until now I'm not able to use the AD7091R-5 with internal reference of 2,5V.
I'm sending this bytes to the AD7091-R to configure it :
meaning config reg 0x02 is set to 0x06C1
-> CMD Mode = 1,
-> Software Reset = 1,
-> Cycletimer to default,
-> P_DOWN = 1, which should enable the internal reference.
config reg 0x01 is set to 0x0F -> All 4 channels on
then set config Reg 0x00

After this I do 8 byte reads where I can see that the conversion results are comming,
the channel index in each word counts also up, but the read values are wrong because
the internal reference is not 2,5 V (I measured around 3,2V at the REFIN/OUT pin)
Related to this "reference" the values are correct, so the AD Converter itself is ok,

But what's going wrong setting the INTERNAL REFERENCE ?

Please help!