W5300 rx_fifor reads garbage data

i am using the W5300 (WIZ830MJ for testing) as TCP server, and i have succes writing and reading all registers except RX_FIFOR.

Transmitting data on the ethernet goes fine and correct data are transmitted.

Recieving goes apparently fine with correct numbers of bytes in the RSR/RSR2 registers, — but data read from RX_FIFOR gives wrong values,
and consistently different values are read after recieving the same datapacket over and over again (only few bytes transmitted to W5300 “CCCCCCC”)

This problem are independant of the socket selected. Can anyone explain why read / write all other registers are correct only except reading RX_FIFOR

Thanks in advance