This error has just started today, it worked fine as recently as yesterday so I don't know if it was caused by a windows update (last night was weekly server maintenance night which includes updates).
This affects both the production server and my workstation where I develop.
The page loads fine but when I click the edit column on the gridview I get this error:
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
After Googling I found this page http://forums.asp.net/p/955145/1173230.aspx#1173230
I have done as suggested there which is to add enableEventValidation="fal
se" viewStateEncryptionMode ="Never" to the page directive but when I do that I get a new error:
Multiple controls with the same ID 'txtStartDate' were found. FindControl requires that controls have unique IDs.
Which is not the case! There is only one control named that, the only other place it is mentioned is for a validatorcontrol which points to this text box as the control to validate.