Just an update on a few other methods I tried after browsing other posts and websites:
Did not work -
1) Turning off antivirus program/firewall before connecting to content server
2) Removing all books from my Calibre library (I don't have many so no problem there), and adding a book that can be downloaded before, to see if any of the newly downloaded books could be corrupted and created this problem
3) System reset on my Kobo, uninstalling Calibre and accompanying files to re-download on my Mac and start over
4) Using Dropbox. Can't finish my login on the Kobo browser, I'm assuming it's because it doesn't support Javascript (? I use a non-Javascript/mobile ver of the content server)
The two following methods worked for me -
1) As suggested by therourke in
this forum, sending books via Gmail and opening the mail on my Kobo's browser (Calibre's email sharing didn't work)
2) As suggested by mylesau in the same forum, using another web server outside of Calibre (I use the Chrome extension which they provided the link of). The newly acquired books can also be downloaded into my Kobo
I enjoy using Calibre to organise my library so I'll continue to use it, but unfortunately cannot transfer books to my Kobo using the content server (for now). I'm relieved to have at least these 2 methods to fall on though.
But if there are any others who would like to share please feel free to do so