The problem is fixed for upgrades to 11.5. Plesk 10.x and 11.0 are affected.
After upgrade from Plesk 8.6, API requests sent using secret key authentication producing errors:
<?xml version="1.0" encoding="utf-8"?>
<errtext>access denied. there are no subscriptions available for this account.</errtext>
The problem is caused by error in upgrade procedure.
Create new secret key after upgrade.