Error when adding post

When I try to add a blog post and click Save I get the following error

Fatal error: Call to a member function return_instance() on a non-object in /home/solal/public_html/perch/core/lib/api/PerchAPI_Form.class.php on line 415

I recently updated the core to version 2.8.6.

Summary information

    Perch: 2.8.6, PHP: 5.3.29, MySQL: 5.0.91, with PDO
    Server OS: Linux, cgi-fcgi
    Installed apps: content (2.8.6), assets (2.8.6), categories (2.8.6), perch_blog (4.5.2), perch_forms (1.8.3)
    App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', 'perch_forms', );
    PERCH_LOGINPATH: /backoffice
    PERCH_PATH: /home/solal/public_html/perch
    PERCH_CORE: /home/solal/public_html/perch/core
    PERCH_RESFILEPATH: /home/solal/public_html/perch/resources
    Image manipulation: GD
    PHP limits: Max upload 100M, Max POST 100M, Memory: 192M, Total max file upload: 100M
    Resource folder writeable: Yes
    DOCUMENT_ROOT: /home/solal/public_html
    REQUEST_URI: /perch/core/settings/diagnostics/
    SCRIPT_NAME: /perch/core/settings/diagnostics/index.php

Any ideas what this could be or how to fix it?