If you in the middle of create a new custom form and find yourself unable to save or edit a data field. You my have a corrupt data field.
To find the corrupt Text box
Hover over each field on the form, and if it does not display its edit button, the field is corrupt and needs to be recreated.
To resolve this issue delete the affected text box and recreating it.
Good Text box - able to see the [Edit] button for the box
Corrupt Text Box - missing Edit button