I have only one user with this problem. He tries to edit an attribute block and when he selects one of the values and erases it the default value doesn't erase. He then has to enter some text and erase that and it works.
If the same drawings work fine on other workstations, then it's not likely a problem with the drawing files.
Has it ever worked correctly for that user?
As a test, create a new Windows user account on that workstation, log into it, and then see if the same behavior can be reproduced there. Alternately, you could have someone else log onto that workstation, thus creating a new local account on the system under which to run the test with AutoCAD.
If the attribute editing works fine under a new/different Windows profile, use the 'Reset Settings to Default' option within the AutoCAD menu grouping on the Windows Start menu (once the original user is logged onto that workstation).
Please report back here the results of the testing and we'll continue on from there if need be. If a sugguestion above clears up the trouble, please mark as a solution so that other users can more easily locate this thread.