Form Builder title/label edit results in INTERNAL SERVER ERROR 500

Hi,

I am getting an internal server error 500 when trying to edit fields (title or label) on existing or new fields in the Form Builder of UM. This error gets thrown by the AJAX call made to admin-ajax.php, as seen in the Code Inspector, PHP debug log is shown below:

POST https://www.kerkopdekaart.nl/wp-admin/admin-ajax.php 500 ()
send @ load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils,jquery-ui-widget,jquery-ui-position,jq…:4
ajax @ load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils,jquery-ui-widget,jquery-ui-position,jq…:4
(anonymous) @ um-admin-field.js:38
dispatch @ load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils,jquery-ui-widget,jquery-ui-position,jq…:3
r.handle @ load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils,jquery-ui-widget,jquery-ui-position,jq…:3

[03-Apr-2017 09:46:53 UTC] PHP Fatal error: Uncaught Error: Cannot use string offset as an array in /home/kerplatform/public_html/wp-content/plugins/ultimate-member/admin/core/um-admin-actions-fields.php:39
Stack trace:
#0 /home/kerplatform/public_html/wp-includes/class-wp-hook.php(298): ultimatemember_admin_update_field(”)
#1 /home/kerplatform/public_html/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters(”, Array)
#2 /home/kerplatform/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#3 /home/kerplatform/public_html/wp-admin/admin-ajax.php(91): do_action(‘wp_ajax_ultimat…’)
#4 {main}
thrown in /home/kerplatform/public_html/wp-content/plugins/ultimate-member/admin/core/um-admin-actions-fields.php on line 39

Does anyone have an idea what could be the cause of this issue?

Regards,
Johan Oldenburg