I had a similar problem a while back (can’t remember when), but it had to do with unsigned and signed bytes and integers…
I think, that IA changed something with Siemens driver and I had to change my Shorts and Ints to Bytes and Words…
My RAMs are over 50 years old and it seems that they have a lot of dead cells… 
EDIT: I found the post with my problem a year ago… mybe it’ll be of some help:
http://forum.inductiveautomation.com/t/siemens-s7-1500-tags-turn-green-after-adding-data/12133/2
http://forum.inductiveautomation.com/t/ignition-byte-tag-config-error/11320/15