There has been an udpdate in the block_admin_list.html.twig in sonata admin bundle. See this commit
If you want to go back to the previous template, download this version of block_admin_list.html.twig and put this file in you app/Resources/view.
Now update your config.yml to change the list_block template :
sonata_admin:
# your config
# ...
templates:
list_block: ::block_admin_list.html.twig
I add the same bug yesterday and this workaround works fine.
Hope this helps