You need to be careful when using Sharing links, if you don't already have access to the folder by default, you have to make sure you always use the original sharing link to access the folder. If someone copied the URL from the folder once you were in it, or you bookmarked that location, you won't be able to edit document or access it never to you use that link because it won't go in via the Sharing security layer.
Thinking this might be getting you cause my users do the same thing sometimes, they will get to the resource then bookmark or copy the URL and ship it to someone else when that is just giving direct link to the location which doesn't use the "Share" link security that was defined.