The action get_header() inserts localization variables before doctype

Confirmed. The SCRIPT element with string definitions comes before doctype, which results in defaulting to other doctype, and breaks the theme.

I’d suggest using jQuery queuing, ie set the variables in a $(document).ready() call, and initialize the functions on $(window).load(), which happens later in the process.

On a sidenote, I’ve edited the “scripts.php” file of the plugin and changed
add_action( 'get_header' , 'bp_like_insert_head' );
to
add_action( 'get_footer' , 'bp_like_insert_head' );

The issues stopped, and the plugin is still working. This hotfix is mentioned in https://www.ads-software.com/support/topic/update-to-019-something-not-working?replies=9

Thanks Sylvaen for the patch. I had the same issue.