If you try to call the Search Express Webservice and you are getting the following exception:

Server was unable to process request. ---> An error has occurred on the server.http://go.microsoft.com/fwlink?LinkID=96177.

Then this hotfix from Microsoft might help: http://support.microsoft.com/kb/946517/

Related to the issue I found the following exceptions in the Application event log of my server hosting Search Express 2008:

• System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
  Event Source: Office SharePoint Server
  Event Category: Office Server Shared Services
  Event ID: 6482
  Description: Application Server Administration job failed for service instance Microsoft.Office.Server.Search.Administration.SearchAdminSharedWebServiceInstance (0e622f50-82da-491b-b986-e21ea759b52a).
• System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
  Event Source: Office SharePoint Server
  Event Category: Office Server Shared Services
  Event ID: 7076
  Description: An exception occurred while executing the Application Server Administration job.
• Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
  Event Source: Windows SharePoint Services 3
  Event Category: Timer
  Event ID: 6398
  Description: The Execute method of job definition Microsoft.Office.Server.Administration.ApplicationServerAdministrationServiceJob (ID e2970977-9408-4ea6-abfa-ad9448a8c626) threw an exception. More information is included below.
  

Another indication for the problem is: If you try to open the IIS Manager you get an error message stating "the path specified cannot be used at this time"

Unfortunately that did not fix the problem completly.

After installing the hotfix I did not get the above mentioned exceptions anymore, but I still got the error message then trying to access the site and the following exception was logged in the application event log:

• Event Source: Windows SharePoint Services 3
  Event Category: Topology
  Event ID: 5617
  Description: The schema version (3.1.8.0) of the database on
  \OfficeServers is not consistent with the expected database schema version (3.1.11.0) on .
  Connections to this database from this server have been blocked to avoid data loss.
  Upgrade the web front end or the content database to ensure that these versions match.

Doing some search on the internet I found the following two sites:

• http://social.msdn.microsoft.com/Forums/en-US/tfsgeneral/thread/53353f69-a007-4250-b395-4ee9dcb95362
• http://social.technet.microsoft.com/Forums/en-US/sharepointadmin/thread/cc9efc81-7ab0-4d85-a5b5-b41685c71ae3

On both sites a form entry suggested to run the following command: "stsadm -o upgrade -inplace -url http://{central_admin_url}"

Running this command got me to the next error:
Upgrade completed with errors.  Review the upgrade.log file located in
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\Logs\Upgrade.log.

In Upgrade.log I searched for the current day and found the following error:
The access control list on C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\template\layouts\web.config could not be modified because the path could not be located in the file system.

So I checked the layouts folder. And suprise there was no web.config file.

After creating a new file named web.config (without content) I re-run the command "stsadm -o upgrade -inplace -url http://{central_admin_url}".

This time it worked perfectly without any errors. SiteCollection and the search webservice are now available again.