Invalid postback or callback argument. Event validation is enabled using in configuration ...

there's a nice explication:
http://msdn.microsoft.com/en-us/library/df6hth3s(VS.80).aspx

BUT you can check if in the IIS is set the correct .NET version. Using 1.1 and having 2.0 set gave me this error when using a custom control with a button in a repeater. I've changed the settings and worked fine, without changing in code.
I've changed back the code, took out the repeater and it worked fine even when the .NET version was not set correct.

SQL user - login error

Data was deleted or changed for security reasons but the essence is the same.
Solution: sp_change_users_login @Action = 'Auto_Fix', @UserNamePattern = 'userx'

Invalid index. (Exception from HRESULT: 0x80200B (DISP_E_BADINDEX))

This is about create a Crystal Report and open it using ASP .NET (C#). The crystal report, when registered ignores those parameters from the argument list which are not defined in the report. When it isn't registered throws the exception shown in the title.
Also verify if in the IIS is set the correct .NET version.