Kristin Cavallari -- Family Loses 'Laguna Beach' Mansion

Kristin Cavallari has some problems back on the home front ... because her family has officially lost their ocean-front mansion featured on "Laguna Beach" after falling behind on mortgage payments.

The house is INSANE -- 5 bedrooms, 5 baths, ridiculous infinity pool, dry sauna, wine cellar, yoga room and some of the sickest beach views you've ever seen in your life.

But TMZ has learned ... the bank recently foreclosed on the property because, according to official documents, KC's father Dennis failed to make payments on the $7,599,458.66 he owed on the place.

And now it's too late for the Cavallari's to get the place back ... because the house was sold at a public auction just last month for $6.1 mil.

Calls to Kristin's rep have not been returned.