On a Citrix Presentation Server with PSE400W2K3R01 installed, Memory Optimization enabled, and the XML service configured as "Sharing with IIS" the XML service gives 500 errors when querying http://<server>/scripts/wpnbr.dll

The IIS log (C:\WINDOWS\system32\LogFiles\W3SVC1):

<date> <time> W3SVC1 <ip> GET /scripts/wpnbr.dll - 80 - <ip> <browser> 500 0 998

Further testing:

In Internet Explorer:
- Tools | Internet Options | Advanced | uncheck Show friendly HTTP error messages
- http://<server>/scripts/wpnbr.dll

This test should return a blank page for a working XML service but returns "Invalid access to memory location."

In the Presentation Server Console:

- Right click on the server farm and select Properties from the context menu
- Select Memory Optimization in the left pane
- Click Add and browse to wpnbr.dll (C:\InetPub\Scripts\wpnbr.dll)
- Click OK
- Click OK to exit server farm properties

Copy wpnbr.dll from a known good source.

In Internet Explorer:
- Tools | Internet Options | Advanced | uncheck Show friendly HTTP error messages
- http://<server>/scripts/wpnbr.dll

This should now return a blank page.

Disabling Memory Optimization at a farm level should also resolve the issue.

The Memory Optimization process runs by default at 3am. This may break applications that appear to be fixed when reinstalled.

The DLL xpperfdata.dll (C:\Program Files\Citrix\System32\Citrix\IMA\xpperfdata.dll) may also be adversely affected by Memory Optimization and may need to be excluded as per CTX109239.

Error : The supplied credentials could not be validated...with Web Interface and XML/IIS Sharing
http://support.citrix.com/article/CTX109239

Configuring the XML Service to Share with IIS on Presentation Server 4.0 and Windows 2003
http://support.citrix.com/article/CTX107683

Explaining and Changing the Citrix XML Service Port
http://support.citrix.com/article/CTX104063

Virtual Memory Optimization Feature
http://support.citrix.com/article/CTX106023