Odd problem today with the XenApp client, erroring each time I tried to login to any published app:
After a fair bit of digging, I found a registry key call “ClientHostedApps” here:
HKEY_CURRENT_USER\Software\Citrix\ICA Client\Engine\Lockdown Profiles\All Regions\Lockdown\Virtual Channels\Control
Simply removing this key and re-opening the client was all it took for the client to operate and the apps to start working again.