Cannot connect to the Citrix XenApp Server - Protocol Driver Error.

Hello there,

When user logs in to Citrix web Interface, using his logon credentials, could log on, but when user to launch the App, following error pops up (attached screenshot):

"Unable to launch your application. Contact your help desk with the following information:Cannot connect to the Citrix XenApp server. Protocol Driver error.

However if I login as administrator on the same PC, I can login successfully and launch app and receive No Error.

Please advise.

Thanks and Regards
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Sekar ChinnakannuStaff EngineerCommented:
Try to recreate the ICA listener in server , for stepl check this article, and copy the httpd.conf file from working server under program files\citrix\XTE\conf and paste it to this server. restart the xte service and check.
Manjunath SulladTechnical ConsultantCommented:
I would recommend you to reboot Citrix server once and check, before you perform below activity.

There could be problem with Session Reliability,

As suggested by Sekar, change the httpd.conf file from working system and restart the service / server.

You can go through belwo Ctitrix thread for more information,
Vijaya Reddy Pinnapa ReddyCommented:
Reboot is the solution. Few days back we got same error. Issue is resolved by server reboot
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Protocol Driver errors can be intermittent, so this may not just be an admin vs. user permissions issue.  

Check your STA configuration, both on the NetScaler and Web Interface.   If you have more than one STA designated, make sure that one STA (XML now as well) is set to primary.  Don't use the STA load balancing.  The primary STA/XML should be your primary zone data collector.
goprasadAuthor Commented:
Surprisingly enough - i have the following scenario.

User access the Citrix published app in windows 7 x 64 bit PC via Citrix Web interface using normal user account,when user clicks on the Published app to launch, Protocol driver error pops-up.

But the same user has no issue, launching published app, if PC logged in as administrator.

This is strange but true, I validated it myself. Not sure what is happening here?

Please advise.
Check your Citrix license server to be sure that valid licenses exist.  Do you have per user licensing?  If so, it's possible that the user license isn't functioning correctly.

A call to Citrix Technical Support regarding licensing issues is typically free.
You might get that problem due to proxy settings on your IE. Try directly connecting to ica instead of Web interface.

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goprasadAuthor Commented:
It's an external PC, cannot connect via ICA, has to go thru via Citrix Web Interface. We are investigating from GPO angle.
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