UltiMaker uses functional, analytical and tracking cookies. Tracking cookies enhance your experience on our website and may also collect your personal data outside of Ultimaker websites. If you agree with the use of tracking cookies, click “I agree, continue browsing”. You can withdraw your consent at any time. If you do not consent with the use of tracking cookies, click “Refuse”. You can find more information about cookies on our Privacy and Cookie Policy page.
I have same problem, cannot retrieve plugins and install octoprint on 3.4.0 and 3.4.1 because of missing ssl libraries, Ubuntu 18 here.
UPD: I was too fast to post, doing sudo apt install libssl1.0-dev solved my issue.