Error when requesting api vk?

Question

API

You need to execute the following method in VK api:
link to the description of the method
To get the service access key, execute the following post request:

$url = 'https://oauth.vk.com/access_token?client_id=6681637&client_secret=мой_ключ&v=5.84&grant_type=client_credentials';

Executed successfully, in response:

(
    [access_token] => 84e7dade84e7dade845ec0a47884822efb884e784e7dadedf6a458d2a70dfa1b02e190b
    [expires_in] => 0
)

But further when executing the query:

$url = 'https://api.vk.com/method/board.getComments?group_id=92037658&topic_id=92037658_37188346&access_token='.$access_token;

The response comes with an error:

[error_code] => 28
            [error_msg] => Application authorization failed: access_token has expired.

The token has expired, how come if the requests are running almost at the same time?