So I've been using QuiteRSS (the current version, 0.18.12) for some time now to keep up with some Tumblr blogs. There were few to no problems up until a few days ago when some feeds did not update. I copied the URL for the RSS feed and pasted it into a new tab in Firefox Nightly. The feed opened just fine.
So I put the same RSS url into a Private window to see if there were any login-related issues. Turns out that's the case. When it loaded, I was redirected to Tumblr's "This Tumblr may contain sensitive media" Safe Mode page (which is on by default), which require the user to log into their Tumblr account to view. I logged in to my Tumblr account in the Private window, went to the same URL and it loaded just fine in Firefox Nightly. Turns out that Tumblr changed something on their side that causes the RSS feeds for NSFW blogs to require a login before they can even be viewed at all.
Because of this, QuiteRSS can't pull any posts from the RSS feeds of NSFW Tumblrs that I follow. Attempting to login via the Authentication tab doesn't work, either -- I put in my username and password, and I still couldn't pull any posts from the affected NSFW Tumblr RSS feeds.
I've contacted Tumblr's support team with details on the issue just over an hour before making this post here. If there is a response, I will update this post. In the meantime, is there a possible workaround for this?
You need to open any news in the embedded browser QuiteRSS. Go to the desired site and login. After that the cookies will be updated and everything will work.
Late response, but I managed to get a page open in the embedded browser, and was able to finally login, get the cookie, and now all of the NSFW feeds work properly again. Thank you so much. Thing is, I had actually attempted this before, but the fields to enter my username and password were so thin that I assumed I just couldn't make it work in that browser. This time, I tabbed between each field on the login page as I entered my details, hit Enter, and was able to login properly to get the necessary cookie.