@ashfurrow Have you tested if your instance really can be accessed over IPv6 for real? Some fetches in Mastodon make a round-trip -- the resource you fetch in turn fetches something from you to verify who you are -- this is where IPv6 connectivity could play a role. If you aren't serving anything from your IPv6 address, you may be able to solve the problem by just disabling your IPv6 address.
@Gargron It appears to be accessible, based on a tool I found online. However, outbound traffic from the machine appears to only have IPv4 connectivity. I'll have to look into how to enable that – maybe it's a DigitalOcean thing. Thank you.