After installing the Update Rollup 12 for Microsoft Dynamics CRM 4.0, I get an error (“Microsoft Dynamics CRM has encountered an error”) in the web frontend when closing an activity (e.g. a task).

The error report says:

<CrmScriptErrorReport>
  <ReportVersion>1.0</ReportVersion>
  <ScriptErrorDetails>
	<Message>Object expected</Message>
	<Line>1</Line>
	<URL>[...]</URL>
	<PageURL>[...]</PageURL>
	<Function>function anonymous()
{
TrySetFocusOnSubject()
}</Function>
	<CallStack>
	  <Function>function anonymous()
{
TrySetFocusOnSubject()
}</Function>
	</CallStack>
  </ScriptErrorDetails>
  <ClientInformation>
	<BrowserUserAgent>[...]</BrowserUserAgent>
	<BrowserLanguage>en-us</BrowserLanguage>
	<SystemLanguage>en-us</SystemLanguage>
	<UserLanguage>en-us</UserLanguage>
	<ScreenResolution>1920x1200</ScreenResolution>
	<ClientName>Web</ClientName>
  </ClientInformation>
  <ServerInformation>
    <OrgLanguage>1033</OrgLanguage>
    <OrgCulture>1033</OrgCulture>
    <UserLanguage>1033</UserLanguage>
    <UserCulture>1033</UserCulture>
    <OrgID>[...]</OrgID>
    <UserID>[...]</UserID>
    <CRMVersion>4.0.7333.2935</CRMVersion>
  </ServerInformation>
</CrmScriptErrorReport>

To fix this problem just delete your temporary Internet files and reload the CRM.