@manueler fix generally worked (thanks a lot!), but for me one additional modification was necessary:
Changing line 1010 from:
if ( in_array( $result[‘owner’][‘id’], $blocked_users_array ) ) {
to:
if ( is_array( $blocked_users_array ) && in_array( $result[‘owner’][‘id’], $blocked_users_array ) ) {
After that i had to resave widgets and it started working.