I think I found the problem.
Directly after the MOVX RDY\BSY' is still high thus giving a false ready signal. So we read when write is still in progress,
this explains the inverted high bit.
I fixed the problem by putting a 1 msec delay after the MOVX.
It takes 4 msec to write so when the delay is over we're in the write sequence and RDY\BSY' has gone low to indicate the write in progress.
My Edit command works Okay now.

 <M> Edit EEPROM
 <1> 0000
 <*>
 0000: .  FF 00
 0001: .  FF FF
 0002: .  FF AA
 0003: .  FF 55
 0004: .  FF  space previous adres
 0003: U  55
 0002: .  AA
 0001: .  FF
 0000: .  00
 FFFF: .  FF
 

Thanks Jon for putting me on the right track