Do you have a customization on the proforma screen? Did you have a duplicate name of a custom field you added to this screen and if so, can you rename or remove it? I've seen this in the past where the custom field name was the same as a global variable and it caused this locking error, so once the custom field was renamed to something else the issue was resolved.

If that was the case then wouldn't everyone get that error when creating proformas that way?  The funny thing is the user received that error but was able to back out of the sales order screen and print it from the proforma screen.

This issue in the past didn't affect every user and the same process was done to get around it. I suggest looking at the screen to see if any new changes were applied.