Quick though

If you are on a distributed installation, maybe you need to re-configure the webpart component to specify the FQDN of the server K2.

HTH

Most likely a Kerberos issue. If you physically log into the server and the tasks show, whilst from the outside it doesn't, you will need to ensure that delegation is configured correctly.

You will also notice that you have user identified as Anonymous in the K2 Server logs when browsing from a client.

can anybody suggest me how i can set the kebros configuration? to resolve the issue

Configuring Kerberos with K2 is a required when running in a distributed environment so luckily there are lots of resources and lots of posts on the K2Underground and on the web in general to help out.  Note Kerberos in my experience is one of those things that seems very complicated and confusing at first until you work with it once or twice and get to understand what is required and why its required.

I would start with the following document:

Security and Kerberos Authentication with K2 Servers

http://www.k2underground.com/media/p/21001.aspx

The getting started guide in the help under SPN also provides everything you need but without all of the explanations in the above whitepaper.

Note to configure Kerberos you'll need someone that has AD Network Administrator privileges to set the SPNs and the Service Account delegation.

K2 Support also has a lot of experience helping customers configure Kerberos so if you get stuck you can give them a call.