Hi @sns
It was probably a problem with one of your lists or list templates. If you have a broken list, then it'll kill the listdata.svc page.
The solution is removing the list that is broken.
you can detect this list so following the steps
1- backup your site
2- try to delete list by list and after delete one test the SPD it will find the that broken
3- restore the site and delete just the broken list
4- don't be afraid when you delete lists take your backup first and the restore is very easy
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