Hello....
Just in case someone faces this issue again...
This is how I fixed it (does not mean it is the best solution)... and so far; so good...
- I created a backup directory .../admin/defs/backup
- Copied all def file into the new backup directory
- Then deleted the troubling file that is indicated in the error message (Links.def) (since the error is coming from deleting a link)
- Went to Setup>SQL Server and clicked [Submit] to redo all default databases (without deleting the data).
- then I went the properties of all tables (Links, Categories, Users ..etc.) and named the fields in the form (the bottom section) as all fields stayed the same but they had empty naming in the FORM.

that seemed to solve the issue..
Thanks to all for help..

Mark