It's no longer possible to sign in on piped anymore
Piped.video wants me to sign in to verify I'm not a bot, but there is no sign in button anywhere. Sad to see a good website fail so badly at the most basic thing.
I thought this was an error given by YouTube to the instances for rate-limiting or something? I usually just swap to another instance in settings and its fixed.
As others have already said that, this is caused by Google/YouTube, not Piped. (They should make that more obvious.)
Try another instance (Preferences -> Instance) until it works. The official kavin.rocks/kavin.rocks (libre) seem to be blocked by YouTube more often than other instances. The smaller the instance, the better (probably).