The page isn't redirecting properly
Pale Moon has detected that the server is redirecting the request for this address in a way that will never complete.
This problem can sometimes be caused by disabling or refusing to accept cookies.
and that's all. The URL of the concerned sites has "tumblr.com/#_=_" in its end in the URL bar. I have no problem to access that sites with other browsers. This behaviour has started to occur after I updated Pale Moon from 27.4.0 to 27.4.1 (from the repository for Debian and Ubuntu).