modbus Unsigned 8 bit

How to read 8-bit Modbus registers? When I try to read a value with a 16-bit unsigned register, I get the error message: Package error 43031: ILLEGAL DATA ADDRESS - Starting address invalid or starting address + quantity invalid
Hey
I found the manual, the register 43031 still should be u16bit. Does any register match the real readings from the unit?
Before it there is a register with bit values. On register 43025 you should use bit encoder/decoder.
I would make sure that you get the communication correct first - so at least some registers are true and without errors. Then test the bit decoder - I think it should still work with 16bit data format.