An error occur when I try upgrading from Zope 2.5.1 to 2.6.0

I have been looking for a specific documentation on upgrading zope 2.5.1 to 2.6.0 but could not find one. I therefore used the Upgrading guide for upgrading zope 2.x.x to 2.x.x . I have manage to install correctly but when import Data.fs form my old Version 2.5.1 to 2.6.0 in the Zope Management Interface try to view the Site Portal it displays me an error.

The error is pasted below.

Your help in any way is very much appreciated.

I thank you in advance for your understanding and help

Thanks

Below is the error message I received

**********************************************************************************************************

Site Error

An error was encountered while publishing this resource.

KeyError

Sorry, a site error occurred.

Traceback (innermost last):

Module ZPublisher.Publish, line 150, in publish_module

Module ZPublisher.Publish, line 114, in publish

Module Zope, line 171, in zpublisher_exception_hook

Module ZPublisher.Publish, line 98, in publish

Module ZPublisher.mapply, line 88, in mapply

Module ZPublisher.Publish, line 39, in call_object

Module OFS.DTMLMethod, line 126, in __call__

Module DocumentTemplate.DT_String, line 474, in __call__

Module OFS.DTMLMethod, line 119, in __call__

Module DocumentTemplate.DT_String, line 474, in __call__

KeyError: css_inline_or_link (Also, an error occurred while attempting to render the standard error message.)

Troubleshooting Suggestions

The URL may be incorrect.

The parameters passed to this resource may be incorrect.

A resource that this resource relies on may be encountering an error.

For more detailed information about the error, please refer to the HTML source for this page.

If the error persists please contact the site maintainer. Thank you for your patience.