Just a techie guy running feddit.online to allow people to communicate, make friends and acquaintances. Odd coming from a happy introvert, right? (https://jerry.hear-me.blog/about)
I also own these publicly available applications:Mastodon: https://hear-me.social/Alternative Mastodon UI: https://phanpy.hear-me.social/Peertube: https://my-sunshine.video/Friendica: https://my-place.social/Matrix: https://element.secure-channel.net/XMPP/Jabber: https://between-us.online/Bluesky PDS: https://blue-ocean.social/ (jerry.blue-ocean.social) Mobilizon (Facebook Events Alt): https://my-group.events/and more...

I've been having the same issue on feddit.online (I'm the owner), where I can log in from the UI, curl, and Postman but get 400 errors from login attempts using any phone app and even Photon and Blorp on the desktop. The devs couldn't help.
However, after waiting 24 hours or so, I was able to suddenly log in.
piefed.zip appears to be, like feddit.online, behind Cloudflare. It would be interesting to see if you can log in after they have Cloudflare entirely clear the cache. There is a potential that Cloudflare is caching a return status. I don't know if PieFed has temporary blocks.