If you want to set a custom edit form to your Sharepoint List then please make sure you follow these Golden Rules as correctly suggested by Hugo in a SharePoint news group.
- NEVER delete or rename the default pages…
when you want to create a new EditForm.aspx it’s best to copy it to
another file and then edit the original one (without renaming it and
without reassigning the supporting files)…
- DON’T ever delete the webparts that exist.
It’s better to hide them and create new ones “side-by-side”.
These rules are real time savers and I will highly encourage every reader to follow them.