Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster - Asp.net, IIS

Machine Authentication Check, or MAC
Machine authenication check (MAC) is there to ensure that the request received from the computer is same as the computer which response transmitted out.

1. ViewState Timeout:

Cause:
This situation is due to shutting down the application pool work process after some ideal timeout.
Solution:
IIS6:

1. Open IIS - run inetmgr
2. View application pools
3. Locate your web application's application pool
4. Right click and Select properties
5. Select performanace tab
6. Uncheck the ideal timeout [shutdown worker process after being idle for...] option

IIS7

1. Open IIS - run inetmgr
2. Click on application pools node
3. Locate your web application's application pool
4. Rightclick and select Advanace Settings
5. Set the Idle Time-out(minutes) property to 0 or I would increate the value to 30 minutes or an hour (60 mins)

For more information please refer this article

2. ViewState Validation Fails

Please refer this article for more information

How to configure directory browsing - II6 / IIS7

How to set up direcotry browsing in IIS 7

1.Open IIS manager - run inetmgr
2.Expand the tree, under [Server Name]/Sites - Click on web site / virtual direcotry required
3.Under IIS section, please click on direcotry bowsing section.

4.On the righthand side please click on Enable/Disable directory browsing option.

How to setup direcotry browsing in IIS6
1.Open IIS manger - run inetmgr
2.Expand the tree, under [Server Name]/Web Sites - right click on web site / virtual directory required.

3.Under Home Directory please check/uncheck Directory Bowsing option