Stuck in loading after submission

The form was working fine for more than a year.
Giving succes message and sending admin emails.

Suddenly it all stopped.

It said the template was outdated so I chose another.
Still it is not working.
After submission it is stuck in loading. No success message… nothing.
I do see the submission showing up in the backend.

Please advice.

Thom

Server Settings

Operating System: Linux (64)
PHP Version: 5.6.35
Server: Apache/2
MySQL Version: 5.6.27
SQL Mode: NO_ENGINE_SUBSTITUTION
PHP Allow URL fopen: On
PHP Max Upload Size: 32M
PCRE Backtracking Limit: 1000000
PHP Max Post Size: 32M
PHP Max Script Execute Time: 60
PHP Memory Limit: 256M
Memory usage: 27.96 MByte
PHP Exif support: Yes (V1.4 )
PHP IPTC support: Yes
PHP XML support: Yes

Graphic Library

GD Version: bundled (2.1.0 compatible)
FreeType Support: Yes
FreeType Linkage: with freetype
T1Lib Support: No
GIF Read Support: Yes
GIF Create Support: Yes
JPEG Support: Yes
PNG Support: Yes
WBMP Support: Yes
XPM Support: No
XBM Support: Yes
WebP Support: No
JIS-mapped Japanese Font Support: No

Active Plugins

ActiveCampaign
Artbees Themes Captcha
iThemes Security
Pixel Cat Lite
Form Maker
WPBakery Visual Composer (Modified Version)
Really Simple SSL
Regenerate Thumbnails
Slider Revolution
Say What?
The Events Calendar Shortcode
The Events Calendar
Yoast SEO
Smush
Subscribe Form by Arrow Plugins
WP GDPR Fix Pro

Active theme

Jupiter Child Theme

The page I need help with: [log in to see the link]