Thanks Erik, you're right (again). Indeed some weeks ago I checked the 'checksum' option in Flashmagic, but as the code is quit large and only a small part is executed during normal operation everything seemed to work fine. Only when the code was shifted by adding some instructions, some parts seemed not to work anymore. I did not relate it to this 'checksum' option. (I also expected it to work differently, more like a 32 bit CRC for all the code in a non used location )
Now I put this option off and the byte at the end of the block is ok again.
Thanks again Erik, this eliminates all uncertainty I had last few weeks.
Also many thanks to everybody who reacted and tried to help.