Fullwidth Easy Slider click to edit image info -> "Error fetching content"

Hello,

I am trying to add a fullwidth easy slider element to my page. This was working previously, but now when I click an image to edit properties, I get a popup saying “Error fetching content – please reload the page and try again”. Reloading the page is not helpful.

For what it’s worth, I am seeing that the call to admin-ajax.php is resulting in an HTTP 500 response code. Below is the form data, from the Chrome Inspector. Note that I recently updated this site to WP 4.7.2, wondering if that explains why it worked before and not now …

Any help would be most appreciated.

POST data to admin-ajax.php:

action:avia_ajax_av_slide_full
params[subelement]:true
params[extract]:true
params[shortcode]:[av_slide_full id='43193' slide_type='' video='http://' mobile_image='' video_format='' video_ratio='16:9' video_controls='' video_mute='' video_loop='' video_autoplay='' title='' custom_title_size='' custom_content_size='' caption_pos='caption_bottom' link_apply='' link='' link_target='' button_label='Click me' button_color='light' link1='' link_target1='' button_label2='Click me' button_color2='light' link2='' link_target2='' font_color='' custom_title='' custom_content='' overlay_enable='' overlay_opacity='0.5' overlay_color='' overlay_pattern='' overlay_custom_pattern=''][/av_slide_full]

params[_ajax_nonce]:5908359dfc
ajax_fetch:true
instance:2
avia_request:true