It's a problem caused by your custom livery made with an earlier version of the Paintkit, which is missing new parameters we added later on. You can fix this by replacing the [Couatl] section of the SIM.CFG for the custom livery you made for the FSDT_Staircase_02_USER vehicle, with this one:
[Couatl]
type = Staircase
cruise_speed = 2 // meters/sec
acceleration = 0.2
front_wheels = 1.7
rear_wheels = 1.47
condition = ( %(exitAlt)f >= 2.60 ) and ( %(exitAlt)f <= 3.50 ) and ( %(isCargo)f == False ) and ( %(isCargoStand)f == False )
animated = 1
anim_lift = 0 200 20
interpIntervals = 0.0,0.5,1
extension = 3.653,4.466,5.279
height = 2.529,3.051,3.571
extensionAdjust = 0
heightAdjust = 0
staircase_start_extension = 0.162, 0.976, 1.789
staircase_start_height = 2.531, 3.05, 3.57
staircase_end_extension = -3.701, -3.701, -3.701
staircase_end_height = 0.317, 0.317, 0.317
In general, if you have several custom _USER vehicles, I suggest checking all of them and be sure the [Couatl] section is identical to the one of the normal objects, so this won't result in losing your repaints.