since, the same issue cannot be produced on Standard AX, you have to make sure
1) if you have customization on this form SalesEditLine
2) if you have customization anywhere on your sales order posting artifacts?
3) is that related to usage data? clear usage data and re-run, sometimes, corrupt memory/data leads to errors
4) compile your classes related to SalesFormLetter***
yes I can open this form from periodic without any issue. you have to make sure, about objects of your AOT to know if there is any customization