LTC2263-12 (DC1532A-L) is not working and can't program all registers via SPI


I am trying to get the 12 Bit ADC on the Evaluation Board DC1532A-L (which houses the LTC2263-12) working with the Microzed Board connected via Microzed FMC Carrier Card. However,  I am reading only two or three different numbers no matter what signal I connect to both inputs of the DC1532A-L. If at any interest: those two numbers are 0b100000000110 and 0b100000000101. The only way to see a different reading was to disable the adc via SPI.

So I thought I would use the Test Pattern functionality that is programmable via SPI on the LTC2263-12. To enable the test pattern one has to write to register A3 and A4, but writing to register A3 and A4 is not effective (which can be seen in the screenshots below).

writing to register A3 the value 0xAA

Suprisingly writing to and reading from registers A1 and A2 works without a problem (the following screenshots show writing to and reading from  register A1):

Thank you.