PHP enotice: is_page was called incorrectly

Adding media to posts and pages seems to have stopped working. The "insert into post" button is greyed and can not be pressed. The following errors seem to feature in my debug.log file:

[07-Oct-2015 13:42:26 UTC] PHP Notice:  is_page was called strong>incorrectly</strong>. Conditional query tags do not work before the query is run. Before then, they always return false. Please see <a href="https://codex.wordpress.org/Debugging_in_WordPress">Debugging in WordPress</a> for more information. (This message was added in version 3.1.) in /var/lib/wp_bilby/wp-includes/functions.php on line 3622
[07-Oct-2015 13:42:26 UTC] PHP Notice:  The called constructor method for WP_Widget is <strong>deprecated</strong> since version 4.3.0! Use <pre>__construct()</pre> instead. in /var/lib/wp_bilby/wp-includes/functions.php on line 3457

Do you know where I could start looking for the culprit to this problem?

Thanks - Mark