I've been seeing this problem sporadically for at least a couple of days.
In one case, it was in a Firefox browser, and it's very much intermittent. My first response was to delete all cookies and quit Firefox, which did fix it temporarily. I later found that simply quitting Firefox and relaunching it would fix it (again, temporarily).
In the other case, it was 100% consistent. It was the 'curl' command-line tool, and it was failing every time with a 403 Forbidden response. My work-around was to use a user-agent string other than "curl/whatever". Even something fanciful, like "Fanciful/99" would cause 'curl' to work.
I retested the aversion to "curl/WHATEVER" by setting Safari's user-agent string to "curl/99", and that led to 100% failures. I also tried some fictional user-agents, like "Bond/007" or "Smart/86", and they worked ok.