Sporadic EncryptException “Could not encrypt the data.” in EncryptCookies

[zeldi-dev]

This question has been also asked on SO, if it is considered cross-posting, please let me know.

We have a CentOS7.5 LAMP server with PHP5.6.37, openSSL 1.0.2, Laravel 5.4. (ATM we cannot yet migrate to PHP7).

Sporadically we receive the following error:

exception 'Illuminate\Contracts\Encryption\EncryptException' with message 'Could not encrypt the data.' in ./Illuminate/Encryption/Encrypter.php:95
Stack trace:
 #0 ./Illuminate/Cookie/Middleware/EncryptCookies.php(131): Illuminate\Encryption\Encrypter->encrypt('KoKVTECWZF4A8x7...')
 #1 ./Illuminate/Cookie/Middleware/EncryptCookies.php(59): Illuminate\Cookie\Middleware\EncryptCookies->encrypt(Object(Illuminate\Http\Response))

The problematic code is:

$json = json_encode(compact('iv', 'value', 'mac'));
if (!is_string($json)) {
    throw new EncryptException('Could not encrypt the data.');
}

I have logged the values, where the program crashes during the night. If I manually rerun the code, I get normal response without an Exception.

$value = 'a51446732955fcf3bb467fc135741ae7956dc247d562b16157de231b8d792400';
$mac = 'Xdo0mGNCeX4p2AHj4cgbmounu3F2bql05uSmve7DDmUWpmMJ1ysmsSrGPg+57N52FxqszqDU5PBTGMihan7ZZQ==';
echo json_encode(compact('iv', 'value', 'mac'));

1{"iv":"yHUQHrhaXq7T+VikdvXuqQ==","value":"a51446732955fcf3bb467fc135741ae7956dc247d562b16157de231b8d792400","mac":"Xdo0mGNCeX4p2AHj4cgbmounu3F2bql05uSmve7DDmUWpmMJ1ysmsSrGPg+57N52FxqszqDU5PBTGMihan7ZZQ=="}

These exceptions happen sporadically, all in the early morning hours (3AM-8AM) , sometimes there are no errors for couple of weeks, then they happen every consecutive day.