Lets first discuss that Plateau at the end, that is, where the SPICE source has pushed the diffpair to an imbalanced condition, because the voltage divider (250K and 5K, or 51:1 division) no longer can move the right-hand base of the diff-pair any higher even as the SPICE source continues to move up.
Cure for this is to reduce the SPICE source maximum voltage to (500 volts - 2*Vbe) / 51 or (500 - 1.2v)/51 or approximately 499/51 or about 9.8 volts.
Now lets talk about that lower end behavior.
The diffpair (used nicely as an error amplifier, or servo-amplifer, or regulator amplifier) compares the input voltage from your SPICE source, to the output of the 250K/5K voltage divider.
To perform this comparison, the diffpair must be used properly, where
1) suitable current flows into the emitters from that single NPN below the emitters
2) the emitter-collector voltage needs to be several volts (examine the datasheet curves)
Lets assume the base voltage into the right transistor of the diffpair must be at least 2 Vbe (2 diode drops, which vary slightly with temperature and with current and with the transistor PARTNUMBER and probably whether the transistor is rated for 6 volts or 60 volts or 600 volts (as in this case) across the emitter-collector pins.
Recall that divider has a 51:1 ratio. You need about 2Vbe, or about 1.2 volts, so the bottom NPN (current source) outputs a steady current.
Thus the minimum voltage coming into the 51:1 divider is going to be
51 * 1.2 or about 60 volts.
To get this down close to ZERO volts Vout, you need to take the advice of someone here, and grab all those Ground nodes (except the 51:1 divider) and run those Ground nodes at about -1 or -2 volts.