SYSTEM LOGS - What does the error mean ?

Log type:  Unknown  
Severity:  11  
Message:  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.  
Exception:  System.Web.HttpException (0x80004005): 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. ---> System.Web.UI.ViewStateException: Invalid viewstate. Client IP: 195.54.163.122 Port: 62618 Referer: http://www.atlantisbt.com/register.aspx Path: /register.aspx User-Agent: Mozilla/0.6 Beta (Windows) ViewState: aayjeXAq85sBYIY3PUk1RoLTQBJn/ LZq8X30l+t xU1uOOlsqydPmW0bFdMaDtDZ7NIFOYE6pokv6zVWPUzyLZFeQOpuBY+C5dV7 0m1GzqbS5BOo+wtlqdk8jVYM JO4e6gzjTwd8yvDE8L+NScLc6LFmKoe0sLHx7Bai4uedJq0KZ9pMb+e+QmzqbhnUtZ8S3ua4Y 1XqXMioF SJtQY0xVoi3zUZXBTD0Ifdg9a1oY tc34gOuU0v/yhAJ+Y0N8AVUhXTRL8ID a/ps7kHI9KYXpIS9zFazL+2HoZIHQoqWiHZrOuY08p m1X0JgzY4r3ADWmMudFcblYUzq5eddZhkszIsnjfJ7y AxC4zf7l7mPzQWzlijldycoJv5Mu0hqFFCt9s+axaKo rqwFr4wcZJM7jkogfPReF UVo2g2AlC1B1uItNEyhhiuEb811WTqf6jP11GKKZ3g ozgCQPBgurM8f8iEsp54Qzad7PW4Hz+HPmuPQhwqib hfuqc/L03yoVCMvN2Cmh24Gdy1e2nuXf0hFnjC5vOCTRn7CTtAV sd3f83+cfmNtraBAVJx81 6qsB8TGQRXd8QGHEXu+Irjn9xsmE5V1EFK/8rJy5+lNaG/vxQnJSa4ItQdKI0IHHZemlGoc4PT9bVdCyYM/JH04C+QnDxS1SdeoKt/LV3mNKA/X3OZXXczEJeWWVUGUu9Rh1sYFJTPWol4HBMc aLNRIbGvoJk/SE2mqJ8DHbuc/2zFzq1B3hvcBaZoV4ogLGFcAOUQ8fm0NO/TbXRw6A0RZTXxCcqKZnvsg/ohtQRlhXqrzbV4dEkVU0aiiLak8VX68NhSiIkw/SWqzwvDC3kbX3Ge... at System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) at System.Web.UI.HiddenFieldPageStatePersister.Load() at System.Web.UI.Page.LoadPageStateFromPersistenceMedium() at System.Web.UI.Page.LoadAllState() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page. ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)  
IP address:  195.54.163.122  
Customer:  
Page URL:  http://www.atlantisbt.com/register.aspx  
Referrer:  http://www.atlantisbt.com/register.aspx  
Created on:  16.04.2011 04:20:18

hello ucakci,

You can read about it in this post:
https://www.nopcommerce.com/boards/t/2857/validation-of-viewstate-mac-failed.aspx

but the short of it is you should add a machinekey to your config file:

Under   <system.web> section add the following:
<trust level="Full" />
<machineKey validationKey='##############'   decryptionKey='##############'   validation='SHA1'/>

Generate the above #### keys using a asp.net key generator on the web.

http://www.developerfusion.com/tools/generatemachinekey/

Thank you , I have applied to my site and wait for next errors :)

I have one more error :

Log type:  Unknown  
Severity:  11  
Message:  This is an invalid webresource request.  
Exception:  System.Web.HttpException (0x80004005): This is an invalid webresource request. at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)  
IP address:  88.225.26.112  
Customer:  
Page URL:  http://www.atlantisbilisim.com/webresource.axd?d=zcd6h-sgimi2xhhf7m'  
Referrer:  http://www.atlantisbilisim.com/category/58-microsoft-windows-7-ultimate.aspx  
Created on:  14.04.2011 08:21:38  

could you please help me ?

Not sure about that one (I get them as well)...
https://www.nopcommerce.com/boards/t/7301/strange-error-message-in-system-log.aspx