After 15 years my washing machine drain suddenly started running over (backing up). I have used drain cleaning solutions to no avail. The vent is about 10 feet away and is shared with the kitchen sink with no problems with draining. What could cause this sudden problem?

After 15 years my washing machine drain suddenly started running over (backing up). I have used drain cleaning solutions to no avail. The vent is about 10 feet away and is shared with the kitchen sink with no problems with draining. What could cause this sudden problem?
If the horizontal run is 10' to the vent stack from the washer, most likely the line needs snaking. The vent stack distance is far, so a cleanout is somewhere near the end of that run. Nm