I don't know how technical you are but this is the regex we use

export const B64_REGEX = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/

Can you dm the cert somewhere? You can also safely share it here as it's just the the public side, but I'll leave that up to you. I suspect there's a small formatting issue because an encoding with one = works for me.

Depending on how you are copying it (e.g. double click), sometimes it won't copy the whole thing.