That seems to have fixed it, @_tilde! I’m now able to pull up your blog @tilde-is-writing@writing.tildelowengrimm.com and follow it from Mastodon.
Otherwise, I’ll dig into why our domain validation failed here. The only potential issue might be DNS – it should be me.writeas.com instead of [username].writeas.com. But could very well be something else. Either way, you should be all set now!
Also, just to note, any current followers of @tilde@write.as will want to re-follow at your new custom domain handle – on Mastodon, they’re two separate accounts.