Various errors, culminating in a requirement of Emacs 28 from org-projectile.
I’m on Emacs 27.2.
Apparently last time I installed from source ([[Installing Emacs from source]]).
Maybe I’ll give that a quick go.
Alternatively I can get it from a Mint PPA.
But that might require me to upgrade to Mint 21 (currently on 20.3).
Ho hum. Things are just about working, so I’ll just leave it for now.
Update: following updating Emacs to 28 ([[Updating to Emacs 28 on Linux Mint]]), all seems to be working again.