Connect Registration Failed #21401 (MK3.9)

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What happened? 

During the configuration of Prusa Connect on the Original Prusa MK3.9, an error on this configuration can occur and the printer shows the message "Registration to Prusa Connect failed due to: ~". In place of ~, a text with more details is shown. 

Error name: Connect Registration Failed

Error code: #21401

How to fix it? 

Follow the instructions in the Connect Registration Failed guidepost, common to our printers with Buddy firmware, with more details on possible causes for this error and their solutions.