Error when posting to a Facebook page as the page-Collection of common programming errors

I am getting an error when trying to post to a Facebook page, as the page.

The page admin has granted Manage Pages permission to the app.

I use the PHP SDK.

I have the user extended token saved, and it is not expired, I already checked that, because when I run /userID/accounts, I get back all the pages this user manages, including each page access token.

I then try to post to one of these pages, using the page access token I just obtained from /userID/accounts. It had been working before, but now when I try, I get a rather cryptic error that I can’t figure out:

FacebookApiException Object
(
    [result:protected] => Array
        (
            [error] => Array
                (
                    [message] => An unknown error has occurred.
                    [type] => OAuthException
                    [code] => 1
                )

        )

    [message:protected] => An unknown error has occurred.
    [string:Exception:private] => 
    [code:protected] => 0
    [file:protected] => XXXXX/src/base_facebook.php
    [line:protected] => 1106
)

Thanks for any help.