The error message is a bit misleading, because what it actually means is that the underlying simulation time step could not be computed, even though it tried progressively smaller timesteps. I suspect the underlying problem is that specific situations cause the models – especially MOSFETs, but also various SPICE models – to behave in a discontiguous/discrete/chaotic manner, which the solver has difficulty dealing with.
In simple terms, you can interpret the error as "this circuit is not simulatable as-is, because it causes (some specific model) in it to misbehave".
The error message you see refers to Q1, which is the 2N7002 MOSFET, so I would suggest you first try adding say a 10Ω resistor to gate of Q1.
If that does not work, my next suggestion would be to move the 10Ω it to U1A pin 3 side, and add another 10Ω resistor to the TX line.
If that does not work, I'd add say 1Ω resistors to the output of each voltage supply.
The resistor values themselves aren't that important; they just ensure that the currents from pure voltage sources stay realistic. For MOSFET gates, it limits the peak current draw when switching states (and thus slightly slows down the switching rate), so I've found it very useful to look at the peak current over the limiting resistor (whatever value you happen to use) to determine whether the circuit is realistic and whatever drives the MOSFET gate can handle the very short peak current draws.
The same advice would apply if the error referred to some SPICE model instead.
(Although I have KiCAD 8 installed, it is Friday evening here, and I'm feeling too lazy to experiment with this myself. Apologies!)