An irregular reminder that calling this place "Mastodon" has a bit similar energy as saying "I just sent you a GMail."
"Fediverse" (or "fedi") is a better term.
Mastodon is just one of many software projects that talk to each other to create this multi-instance social network:
https://axbom.com/fediverse/
(kudos to @axbom for this fantastic infographic!)
There are blogs, Reddit-like communities, video instances and many more, all part of fedi.
I know approximately nothing about internals of that plugin. From what you're saying ("couldn't generate a valid URL" is very vague and doesn't tell me what context did that happen in) I would wager that there's some confusion around the domain that should be represented by the blog (i.e. the domain in the blog's @something@domain address).
@robryk For some reason the WordPress plugin didn't work for my WordPress Multisite setup. I have a blog on a subdomain https://codecorner.galanter.net - and it couldn't generate a valid URL