Clients who haven't been seen after 150 days (that's aprox. 5 months) loose their credits on your aMule.
The sentence should be "Creditfile loaded, 7550 clients are known. Credits expired!" but there's a typo, as you can see (it's fixed in future rc4). So, what that means is that there are 7550 clients known (7550 clients who have credits on your aMule) and that some clients just lost their credits on you, so, last time you booted aMule before this time, you had more than 7550 clients known. Just don't care about that, it's a wirning for development basically ;-)
Greetings.