TypeError: Cannot access offset of type string on string in Drupal\eca_views\Plugin\Action\ViewsSetFilter->execute() (line 68 of /var/lib/tugboat/stm/web/modules/contrib/eca/modules/views/src/Plugin/Action/ViewsSetFilter.php).

The problematic line is:
$event->getView()->filter[$id]->value['value'] = $value;

Not sure if this is the right way to fix it, but the error doesn't show up anymore if we check if $event->getView()->filter[$id]->value is an array before trying to use it as an array:
Replace line 68 with

      if (is_array($event->getView()->filter[$id]->value)) {
        $event->getView()->filter[$id]->value['value'] = $value;
      }
      else {
        $event->getView()->filter[$id]->value = $value;
      }  

It's a bit more complicated than this, unfortunately. There can be simple fields (e.g. the taxonomy name), or complex fields like a date or the taxonomy term, which provide their filter value in an array, using the key value for it. But there can also multi value filters, e.g. when using the operation "Is one of", then the filter array is indexed by integers.

I have committed a fix to the 3.0.x branch, please review and test that one.