Unable to add Drupal 7 sites to DRD 4

I've successfully added 10 Drupal 8 cores with a total of 47 domains, with nine of those cores hosted on Acquia Cloud. We have three additional Drupal 7 cores hosted on Acquia Cloud, with 21 domains between them. I can't successfully add any of those.

I can successfully create the core, and the list of domains is successfully retrieved.

When I try to authorize DRD access, the DRD Agent running on the target site logs "Can not read input".

DRD itself logs (for example):

drd_action_info [drd_domain/956]: Success with response 
null
null

All domains are using the same set of firewall rules.

I'm not certain if this is related, because I can only get it to happen on my local dev environment, but when I clone my Dashboard site to my local and try to add the D7 cores, I get the following notices:

Notice: unserialize(): Error at offset 15789 of 72392 bytes in Drupal\Component\Serialization\PhpSerialize::decode() (line 21 of core/lib/Drupal/Component/Serialization/PhpSerialize.php).

...and:

Notice: unserialize(): Error at offset 18262 of 491915 bytes in Drupal\drd\Crypt\Method\OpenSSL->decrypt() (line 187 of modules/contrib/drd/src/Crypt/Method/OpenSSL.php).

(There's a fair amount of json coming out with this, which I can provide if it'll be helpful.)