@devnull They can be a bit picky. I remember fixing an interop issue by removing the space after the commas in my HTTP signature header. I.e. before:
keyId="...", algorithm="rsa-sha256", headers=...
After:
keyId="...",algorithm="rsa-sha256",headers=...
I don't remember with 100% certainty whether it was Misskey that needed this.
FWIW, the grapevine tells me that the upstream Misskey team can be difficult to reach, but I've experienced the Iceshrimp team to be very friendly and helpful.