Unfortunately this is happening again for me today and the solution that worked before, deleting the URC folder, is no longer working.
I also tried to run Epic games as administrator, but that did not solve it.
It is preventing me from working on maps. 
I also just noticed that on these forums, even when I am logged in and posting here right now, the top right is asking me to “Sign in”, if I click on it, it sends me to the homepage and says I am logged in, but then I navigate to the forums and this thread, the top will still display “Sign in”. But, as you can see I am able to post.
I am wondering if I should make a new post for this issue… but I am REALLY wondering if it is related to my “Cannot Sync… not logged in” within UEFN.
Update: I am able to use UEFN and my project now loads and it seems it is able to properly connect to the content server. This now works for me after uninstalling virus protection software (malwarbytes)