Maybe it is helpful for other users, but I was able to solve this error by editing capa.php as follows. No idea whether the plugin is fully functional after this edit, but it solved my issues.:
…
$category_taxo = capa_protect::get_value_categories(TRUE);
// ADD LINES BELOW
if ($_terms instanceof WP_Term) {
return $_terms;
}
if(!$_taxonomy){
…..