Last week I moved the backend of my db on to our sql server, using append queries I moved all of the historical data to the proper tables....
Now the users cant edit any data that has been added after the move to sql server. They can however edit data in the same table prior to the move. The error was something along the lines of "the record is being edited by another user do you want to drop your changes or copy them to the clipboard".
There is no need to use an ADP file with SQL if you don't want to, especially with the number of users you are talking about. I quite happily run an Access 2000 database with a SQL Server 2000 database using ODBC.
Try deleting and then re-linking all of your tables within the Access front-End. When access prompts you to "Select Unique Record Identifier' don't just press 'Cancel' - u must select a field here if you want your data to be read/write.