OK, think I have worked out what is happening.
I have WordPress running in a folder https://domain.tld/wp/ and when I try and mark posts as unresolved/resolved etc the url strips the slash before the ?action… eg: domain.tld/wp?action…
If I copy/past the url and add a slash it works eg: domain.tld/wp/?action…
A quick fix, in p2-resolved-posts.php around line 338, I changed get_site_url() to just site_url(‘/’) and then everything works.
$link = add_query_arg( $args, site_url('/') );
return $link;
I think this has something to do with folders? As it seems to work on a top level – Is this safe/ok?