The trailing slash error (afaik) can't be removed just by entering the same origin again. In my case, unsuccessfully, I removed my remote, then added it again: 
git remote remove origin  
git remote add origin https://github.com/andybywire/jekyll-foundation.git
But when I tried to push my changes, I still got the error:
fatal: unable to access 'https://github/andybywire/jekyll-foundation.git/': Could not resolve host: github
... trailing slash intact. After some further investigation I noticed that some of my other repos leave the .git off altogether:
git remote -v
origin  https://github.com/andybywire/content-first-prototyping (fetch)
origin  https://github.com/andybywire/content-first-prototyping (push) 
When I removed the .git from the first repo, the error went away entirely: 
git remote remove origin  
git remote add origin https://github.com/andybywire/jekyll-foundation
This repo now allows me to do everything I would expect. 
I don't know why, but I also went round and round with this (restarts, proxy settings, toggling wi-fi); this is what worked.