Starting with Citrix Workspace App version 20.9, adding an account on the Workspace app gives an error "Cannot verify this server's certificate: ***********. The server might not be secure. To continue access, tap Setup and check "Ignore certificate warning"

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To fix this, a cert with the SHA2 algorithm is recommended to be implemented on the storefront. Alternatively, the users would have to manually click Setup and select “Ignore certificate warning on their iOS devices.

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Problem Cause

As per https://support.apple.com/en-in/HT210176 Apple states for iOS 13 requirements: 

TLS server certificates and issuing CAs must use a hash algorithm from the SHA-2 family in the signature algorithm. SHA-1 signed certificates are no longer trusted for TLS