86400 is the number of seconds in 24 hours.
Is there a stop time built into the model? This could be done in a number ways:
There may be a stop time defined in the main run panel at the
top of the interface. Press the down arrow to the right of the run time
to see if there is a stop time defined:

There may be user event that stops the model (see Toolbox > Modeling Logic > User Events).

There could be a message trigger that stops the model, where a delayed message was sent on reset.
Some other trigger could be fired at particular times that stops the model, like a time table.
To determine if it is one of these latter two possibilities, search your model for "stop(":

If you still can't find a reason the model stops, check the Event List. Do so by the following:
- Open FlexSim
- Open your model
- Set a stop time (see how to do that from the run panel up above) for time 84599.9
- Run the model until it hits your set stop time, shortly before the rogue stop time
- Open the Event List from FlexSim's main menu > Debug > Event List
- Look at what events are scheduled at time 86400
- Figure out which one stops the model
You didn't mention this, but at time 86400 is it not that the model has stopped, but actually it has crashed/frozen? Is FlexSim still responsive? If not, you could follow these steps for debugging the events that might be crashing your model.