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I’ll write here since you don’t seem to check the threads on the Materialis Companion page.
In the file wp-content\plugins\materialis-companion\theme-data\materialis\integrations\gutenberg\integration.php line 29 you are queuing gutenberg-style.min.css using get_stylesheet_directory – this causes an error when using a child theme.
The work around is to copy wp-content\themes\materialis\gutenberg-style.min.css to the child-theme.
Another thread regarding the issue: https://wordpress.org/support/topic/help-warning-message-after-update-plugin/