Hello.
Message Warning: htmlspecialchars(): Invalid multibyte sequence in argument in check_plain() (line 1670 of /var/www/vhosts/4/112314/webspace/httpdocs/totapodroz.pl/includes/bootstrap.inc).
That above was my problem like probably many of you. My reason was litlle different that I've found on the internet.
I've got error above when was 2 conditions:
*node has field with AddThis_field
*node title has special polish signs (ąęćżźół)
After this solution https://www.drupal.org/node/1430166#comment-6647386 the problem gone! :)
/in my case it was line 1670 but the function is the same/
Maybe it is possible to implement this to core?