When testing remote access to Citrix applications using an iPhone, I received this very helpful error message when I tried to connect with the Citrix Receiver:

In this case, my Citrix environment is front-ended by a Citrix NetScaler and Android devices worked with no problem. After doing a quick search, the following was found on Richard Parmiter’s blog:

After much messing about, it seems that when creating a XenApp service site on the Netscaler, a checkbox is provided stating “enable connection through mobile receiver”. When this is selected a few “rewrite” rules are created to resolve a problem with the iPad/iPhone receiver connections but the rewrite feature is not enabled at the same time on the Netscaler. Simply right clicking the “rewrite” menu in the left panel and selecting to enable the feature is enough to resolve the problem.

After doing this, all mobile receivers can connect as expected.”

Sure enough, the Rewrite feature on my NetScaler had not been enabled. Once enabled, iPhones and iPads were able to connect.

Be sure to save the configuration!