Former Evanescence guitarist Ben Moody has 500,000 reasons to be in a piss poor mood ... 'cause his managers are suing him ... claiming he owes half-a-million bucks in unpaid commissions.
FYI -- Ben's also a HUUUUUGE songwriter ... penning hits for music giants like Celine Dion, Avril Lavigne, Daughtry ... and he even wrote the Kelly Clarkson mega-hit "Because of You."
Also ... he produced a couple of songs for Lindsay Lohan (but hey, everyone makes mistakes).
Anyway, Ben's being sued by Rider Management Inc. ... which claims they inked a deal with Ben back in 2006 (3 years after he left Evanescence) to receive commission on his various projects.
But according to Rider ... Ben stiffed the company out of $500,000 in back commissions and kept the money for himself.
We reached out to Ben, but haven't heard back ... which is sad. What's even sadder ... SOMEONE LET LINDSAY LOHAN COVER A STEVIE NICKS SONG?!?!