Can’t modify info in “Additional CSS” section

Hello,

I have recently noticed that it is not possible to save changes in “Customize/Additional CSS” section. When I try to do it, these errors appear:
– Failed to load resource: the server responded with a status of 406 ()
– POST https://my-site.com/wp-admin/admin-ajax.php 406

and there is a pop-up message:
Looks like something’s gone wrong. Wait a couple seconds, and then try again.

I am not sure if the reason of this issue is Jetpack plugin, but I Googled about it and found that some people solved it by deactivating Jetpack. In addition, this issue appeared last days, together with plugin update.

I don’t want to deactivate Jetpack because I need this plugin. Can you please check if it can be the reason of such behaviour and how to fix it?

I use Storefront parent theme and Galleria child theme.

Thank you,
Nick