I just re-actived my BONIC account and added some machines for it. However, I found recently the credit I got was negative...How can it be?

Because you haven't contributed in a long time, one or more of your projects has decided that you need a new cross-project ID. This is the identifier that's used to track your stats (and other things) across projects. In order to get it synched up, you'll need to attach at least one host (you don't need to complete any work, just attach so it shows up in your hosts page on that project's site) to every project you've ever had credit for. Then, it may take up to a couple of days for the changes to take affect and be shown on the various stats sites. I know it sounds a bit daunting, but once everything gets lined up you should be okay.

The sticky wicket is for projects that have retired or are no longer active. If you were a user of the BOINCstats Account Manager prior to a project being "officially" retired here at BOINCstats then you will be able to manually merge the old CPID with your new one. If not, you're unfortunately out of luck at this time--this is done on purpose so people can't steal someone else's credit. If you haven't computed in a while, chances are you'll be able to regain that lost credit fairly quickly anyway.

It's that Willy is secretly using Enron accounting techniques...