Troubleshooting PHP Notice: Unknown: 1 … where’s the unknown token?

I’m getting a PHP error which I need help to troubleshot:

[Fri May 08 10:20:41.234655 2015] [:error] [pid 6969] [client 207.183.229.82:3038] PHP Notice: Unknown: 1. h->opened_path=[null] h->filename=[/var/www/example.com/htdocs/wp-admin/network/site-new.php]n in Unknown on line 0, referer: http://example.com/wp-admin/network/site-new.php

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This occurs on a fresh installation of WordPress when I try to add a new site.

But what’s the unknown token and why is it unknown? I’ve looked at line 0 of the file wp-admin/network/site-new.php, and it merely contains the start of a php tag: <?php

Additional Info

This error arises when I POST to a WordPress action /wp-admin/network/site-new.php. This attempt actually adds 243 messages to my error log. Here are the first three:

[Fri May 08 10:20:41.234655 2015] [:error] [pid 6969] [client 207.183.229.82:3038] PHP Notice:  Unknown: 1. h->opened_path=[null]  h->filename=[/var/www/example.com/htdocs/wp-admin/network/site-new.php]n in Unknown on line 0, referer: http://example.com/wp-admin/network/site-new.php
[Fri May 08 10:20:41.234716 2015] [:error] [pid 6969] [client 207.183.229.82:3038] PHP Notice:  Unknown: apc_cache_find [1627723]n in Unknown on line 0, referer: http://example.com/wp-admin/network/site-new.php
[Fri May 08 10:20:41.235364 2015] [:error] [pid 6969] [client 207.183.229.82:3038] PHP Notice:  Unknown: 2. h->opened_path=[/var/www/example.com/htdocs/wp-admin/network/site-new.php]  h->filename=[/var/www/example.com/htdocs/wp-admin/network/site-new.php]n in Unknown on line 0, referer: http://example.com/wp-admin/network/site-new.php