Hello dartiss,
I also have the same error.
It displays it in the admin panel->ManFisher->footnore->preview.
Also on the frontend, some things work and some don’t.
– The shortcode tag get removed and replaced by the counter number, but the text between them stays in the text body.
– The hover doesn’t work.
– The summary at the bottom get’s displayed properly.
– but hyperlink doesn’t work, it’s not ever act as button.
Currently I’m using [f][/f] as shortcode, but also tried other ones, with no success.
The plugin was working fine a couple of months ago.