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Solver abruptly stopping/refusing to start

Hi,
 
This will be a horribly vague question, but I'm stumped. Here's the situation...
 
  1. Full Tractor-Trailer model in A/View 2021.4.
  2. This model has flex bodies, contacts, tires, etc., and has run successfully for years.
  3. That is until some recent changes that included new flex bodies.
  4. I'm running these with the external solver as I have for years, but this one abruptly stops as soon as the solver tries to start. End of the .msg file below...
 
 Begin Simulation
 
 ****** Performing Dynamic Simulation using Gstiff SI2 Integrator ******
  The system is modelled with SI2 DAEs.
  The integrator is GSTIFF, CORRECTOR = modified
  Integration error =  1.000000E-01
 
No error message, nothing. "debug/eprint" didn't show anything either.
 
I feel like I've seen this in the past, and it's an obvious (and dumb) error/mistake I'm making. I can't for the life of me remember what it is. Here's what I've tried/checked, all to no avail...
 
  1. Recreated the flex bodies.
    1. They work fine in a smaller test model.
  2. Made sure my .rdf files were present.
    1. Also tried different road files.
  3. Made sure my .tir files were present.
  4. Wrote the model out as a .cmd, read it back in to a new database.
    1. Read in with no errors.
  5. Double-checked my memory settings - all set to "huge".
  6. Looked for parts with no mass.
 
I could go back to the previous iteration and redo my changes, but it took a while and I really feel I'm missing something silly.
 
Any vague ideas regarding this vague question?