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As far as I know Citrix Receiver is End-of-life (except for 4.9 LTSR): https://www.citrix.com/support/product-lifecycle/milestones/receiver.html

EOL, i understand, and if it was up to me, all our end users would be on the latest version of workspace. But we have a lot of end users who are on devices managed by third parties. So while prior to 4.9 may be EOL, i was wondering if there was a technical reason for their connections failing. All our clients support TLS 1.2, since we enforce that on the netscalers, so the versions these users are running are not a lot of revisions behind 4.9.

VAD 1912 page states 4.9 LTSR is the only supported version of Receiver. (Plus any supported version of Workspace). No idea on the technical reasons, but try this: https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/downloads/citrix-receiver-feature-matrix.pdf and this https://www.citrix.com/content/dam/citrix/en_us/documents/data-sheet/citrix-workspace-app-feature-matrix.pdf

Are you allowing UDP at all, directly or via NetScaler? Usually the first thing I try turning off when looking at issues like yours.

what version are those clients having issues? I will try to get a technical reason for you if you can give me those details.