Elon Musk -- Second Marriage Kaput ... Wife Files For Second Divorce

Elon Musk -- the brain behind Tesla and PayPal -- has a problem he can't seem to solve -- his wife has filed for divorce ... again.

Actress Talulah Riley has filed to end their second marriage ... a second time. According to the couple, they've been living separately for the last 6 months.

Their history is a love story set in the middle of a hurricane. They married in 2010 but divorced 2 years later. They remarried 18 months later, in 2013. Elon didn't think the second time around was right, so he filed for divorce on New Year's Eve 2014, but withdrew the petition in June 2015.

Apparently it was not meant to be. They say they'll remain friends.