Cannot save Style

Hi
I have Joomla 5.1.0 installations (many). On all, i cannot save under Options a Display Style. This not only witch slideshow. The same problem is with slider.
When i will save, i have an error "failed".
It's strange - this is on all sites that i have.
Slider's and show's will work on life site. Only i cannot save a custom style.
I have at time the free versions of ck.

Any help here?
Many thank's

Hi
this could be due to a server issue
can you please contact me by email with an admin access so that I can check the problem to find the cause
www.joomlack.fr/en/contact
CEd

i have activated the debug in joomla. see the image attached with the error...

please install this fix and tell me
www.dropbox.com/scl/fi/si2nt8kdry75h151j...t5r4zkpfgnrkvhy&dl=0

no - will not work with the fix...
i have sent you an admin-login in mail.

for info after email discussions, the problem seems related to mod_security on the server