You have chosen not to trust… – Citrix Receiver and SSL error 61 on Ubuntu

After months of trouble-free operation, Citrix Receiver decided to wreak some havoc one morning last week.
Connecting to work (using Firefox on Ubuntu and Citrix Receiver for Linux 13.8) was trouble free as usual.
However, when I then tried to select a PC to remote into, Citrix informed me that …

“You have chosen not to trust Entrust Root Certification Authority – G2. SSL error 61”

At that point, I reflected that what I knew about Citrix and SSL certificates would fit on the back of a fag packet.
After some intensive “research” it should now fit into a short blog post…
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