Thank you for your post.
If everything is manual, then there should be no reason why the field cannot be edited, unless you have an Access Privilege set up for a web account.
Try removing the Username field from the layout, and then add it back again (do not cut and paste). It may be the layout is corrupt and Instant Web Publishing is having difficulty making changes to that field.
Please let me know if you continue to have difficulty.
Have you tried going back to original thought of having this automatically entered, but using an auto-enter text field instead of a calculation field? This should work great, the user won't have to do double-entry, and the field can remain a text field.
I have had this same problem.
I first thought it was a privlege issue, so tried logging in as the default full access user, problem still persisted.
The DB will accept direct data entry, that is data typed into the field.
The issue is that the DB will not except data selected from a drop-down list, using a value list, as a data selection and entry command. I even included a commit records script command on the form. However still a no-go.
PS - The drop-down list tests fine in the data base mode. It just does not work in the IWP mode.