Came here looking for how to deal with the certificate error and directly found that it has already been fixed. Installed 2.5.1, and certificate errors are gone. Great job! 
Quick report: After installing 2.5.1 via ./Joplin_install_and_update.sh --prerelease and restarting Joplin, my layout was lost (the panels for the outline-plugin and the note tabs plugin are now vertical at the right side, i.e. their initial unconfigured positions). I think I upgraded from 2.3.9 (not sure).
The problem might also have another reason - just wanted to mention it here.