Version 2.6.7 breaks Widget editor

On version 2.6.7 I can’t save the widget editor anymore.
It only happens when Countdown Timer Ultimate is active on version 2.6.7.

The error is shown as a toast message upon saving saying:
there was an error the response is not a valid json response

Happens when clicking update on /wp-admin/widgets.php

Here is the relevant site info:

` wp-core

version: 6.8.2
site_language: nl_NL
user_language: en_US
timezone: Europe/Amsterdam
permalink: /%postname%/
https_status: true
multisite: false
user_registration: 0
blog_public: 1
default_comment_status: open
environment_type: production
user_count: 26
dotorg_communication: true wp-paths-sizes

Advanced Custom Fields PRO: version: 6.5.0.1, author: WP Engine, Auto-updates enabled
Antispam Bee: version: 2.11.8, author: pluginkollektiv, Auto-updates enabled
Contact Form 7: version: 6.1.1, author: Takayuki Miyoshi, Auto-updates enabled
CookieYes | GDPR Cookie Consent: version: 3.3.3, author: CookieYes, Auto-updates enabled
Countdown Timer Ultimate: version: 2.6.6, author: Essential Plugin (latest version: 2.6.7), Auto-updates disabled
Fiascode Framework: version: 0.1.0, author: Jan Willem Kolkman, Auto-updates disabled
GA Google Analytics: version: 20250326, author: Jeff Starr, Auto-updates enabled
Jetpack: version: 14.9.1, author: Automattic, Auto-updates enabled
Maintenance: version: 4.18, author: WebFactory Ltd, Auto-updates enabled
MetaSlider: version: 3.100.1, author: MetaSlider, Auto-updates enabled
Paytium: version: 4.4.16, author: David de Boer, Auto-updates enabled
Proxy Cache Purge: version: 5.2.2, author: Mika Epstein, Danila Vershinin, Auto-updates enabled
Wordfence Security: version: 8.0.5, author: Wordfence, Auto-updates enabled
WP fail2ban: version: 5.4.1, author: Charles Lecklider, Auto-updates enabled
Yoast Duplicate Post: version: 4.5, author: Enrico Battocchi & Team Yoast, Auto-updates enabled
Yoast SEO: version: 25.7, author: Team Yoast, Auto-updates enabled wp-plugins-inactive (1)

Liveblog: version: 1.9.6, author: WordPress.com VIP, Big Bite Creative and contributors, Auto-updates enabled wp-media