Figured it out! With thanks to this Error "code length overflow" ""solved""? · Issue #52 · davidshimjs/qrcodejs · GitHub
I finally realized it always happened when the encoded string contained non-US ASCII characters (p.e. á, ñ, í…)
This issue was that my title (the one set at Login | HSTS Redirection Community) did indeed contain non-ASCII characters. I guess it was pulling that into the QR code when generating and it choked.
The clue was in site.com/logs and after selecting the most recent log entry, under the info tab, there was a line that read:
RQRCodeCore::QRCodeRunTimeError (code length overflow. (1787 digits > any version capacity))