Frontend HTML – block cannot be updated

I am not able to edit the frontend HTML or PHP.
Any changes are lost after saving.

what I tried:
– Chache turned off
– tested with draft or public Block

update:
it is apparently the php code. only html goes

Wordpress:
WP Version : 5.3.2
MySQL Version : 5.7.28
PHP Version : 7.4.2

all plugins:
WP Hide & Security Enhancer 1.5.9.3 https://www.wp-hide.com/
All In One WP Security 4.4.2 https://www.tipsandtricks-hq.com/wordpress-security-and-firewall-plugin
AmazonSimpleAdmin (ASA1) 1.4.7 https://www.wp-amazon-plugin.com/
Autoptimize 2.6.1 https://autoptimize.com/
Broken Link Checker 1.11.10 https://www.ads-software.com/plugins/broken-link-checker/
Classic Editor 1.5 https://www.ads-software.com/plugins/classic-editor/
Contact Form 7 5.1.6 https://contactform7.com/
Custom Post Type UI 1.7.2 https://github.com/WebDevStudios/custom-post-type-ui/
EWWW Image Optimizer 5.1.4 https://www.ads-software.com/plugins/ewww-image-optimizer/
GP Premium 1.9.1 https://generatepress.com
Lazy Blocks 1.8.2
Redirection 4.6.2 https://redirection.me/
Shariff Wrapper 4.6.4 https://www.ads-software.com/plugins-wp/shariff/
Simple Google reCAPTCHA 3.0
UpdraftPlus – Backup/Restore 1.16.21 https://updraftplus.com
WP Dashboard Notes 1.0.9 https://www.ads-software.com/plugins/wp-dashboard-notes/
WP Edit 4.0.4 https://wpeditpro.com
wpSEO 4.5.6

• This topic was modified 4 years, 10 months ago by yves779.
• This topic was modified 4 years, 10 months ago by yves779. Reason: new test