I ams also facing the similar issue with 7603, for me in Change Module and the message is as follows..
"ARERR  You do not have write access to field : PhaseManagementValidation"
not able to track anything in AL or FL logs..
write Access Permission for this field belongs to "General Access". I checked in the User Form and I could see the General Access in User's Group List.
1. Open the form CHG:Infrastructure Change on Dev Studio.
2. Hit "Control F" buttons on the key pad of your computer.
3. Select "By Type" = Name and search for "PhaseManagementValidation".
4. You will find a hidden field and open the properties.
5. Go to the Permissions.
6. Check if "Allow Any User to submit" is set to "No"? change that value to "Yes" and save the form.
7. Re-test the issue again.
This is related to a defect SW00369907 in 7.6.03 version.