This YouTube error message had been bothering me for months:

An error occurred. please try again later.
Learn More
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgXLvF2KMe-YfIt0QOXnXnQUqhr6_KUQhDunRD43reaf8TtY4x4liHue0OhJNYHHRw_DMQwo2R0lUxxCeEmJOzJyt9UZJcmli5YdUD5uOpJFBE6bPZs5_rErSBHNglZ3VMcJPR5uW-SPoc/s800/YouTube%2520An%2520error%2520occurred.%2520please%2520try%2520again%2520later.%2520Learn%2520More.png

The iconic error screen on YouTube

It turns out that YouTube needs at least one of RC4 options, see about:config and search for “rc4”. I just randomly picked one from the six options, which was security.ssl3.rsa_rc4_128_sha. Reloaded the page and the video played. It is required for both Flash and HTML5 video playbacks, and it doesn’t matter if you are signed in or not. Originally, I thought signing out would help, but that doesn’t seem to be all the cases.

Funny story is that those RC4 options was intentionally disabled last year by me in Firefox. I couldn’t really tell if the playback issue surfaced right after I did so, but I knew it had been going on for long time and I never made the connection between two.

I tried starting up without any add-on enabled, cleaning up cookies and caches, nothing would work. Glad that I never did a full reset of Firefox. Yes, that might make me realize RC4 is the key, but I might not if I do too many tasks at once to get Firefox back to what I would like.

This solution was post on May 9, so I only suffered another month before I finally got the right answer. But I will still not re-enable RC4 permanently just for watching videos on YouTube.