Non-Gurobi Solvers not finding solution

Checklist

• I am using the current master branch
• I am running on an up-to-date pypsa-usa environment. Update via conda env update -f envs/environment.yaml

The Issue

When solving, if I use Gurobi an optimal solution is found on a simple electricity only model. However, if I use other solvers (Ive tested with Highs and CPLEX) I get an unknown termination condition. See below for the outputs from each solver. This error happens for sector networks as well

Linopy writes the LP okay, with the following output

Edit: I think the CPLEX error has to do with the python path, and its defaulting to the community edition then just getting killed cause the problem is too big. But gotta double check that.

linopy log

INFO:linopy.model:Solver options:
 - threads: 4
 - lpmethod: 4
 - solutiontype: 2
 - barrier.convergetol: 1e-05
 - feasopt.tolerance: 1e-06
INFO:linopy.io:Writing objective.
Writing constraints.: 100%|█████████████████████████████████████████| 31/31 [00:01<00:00, 19.83it/s]
Writing continuous variables.: 100%|██████████████████████████████████| 8/8 [00:00<00:00, 38.21it/s]
INFO:linopy.io: Writing time: 1.93s

Gurobi Output

  46   5.20600204e+09  5.20600200e+09  1.90e-07 7.84e-05  5.67e-05     8s
  47   5.20600201e+09  5.20600201e+09  4.49e-08 3.86e-05  7.85e-07     8s

Barrier solved model in 47 iterations and 7.68 seconds (11.75 work units)
Optimal objective 5.20600201e+09

INFO:linopy.constants: Optimization successful: 
Status: ok
Termination condition: optimal
Solution: 344624 primals, 890676 duals
Objective: 5.21e+09
Solver model: available
Solver message: 2

CPLEX Output

INFO:linopy.solvers:Unable to save solution file. Raised error: CPLEX Error  1217: No solution exists.
WARNING:linopy.solvers:Solution status unknown. Trying to parse solution.
WARNING:linopy.constants:Optimization failed: 
Status: unknown
Termination condition: unknown
Solution: 0 primals, 0 duals
Objective: nan
Solver model: available
Solver message: Unknown status value

WARNING:__main__:Solving status 'unknown' with termination condition 'unknown'

highs Output

  55   9.24e-12 1.96e-08   6.65831968e+09  6.65550984e+09  3.91e+00     741s
  56   9.26e-12 6.11e-09   6.65773024e+09  6.65668218e+09  1.46e+00     750s
  57*  1.46e-11 3.73e-09   6.65746563e+09  6.65700661e+09  6.39e-01     759s
Summary
    Runtime:                                            759.21s
    Status interior point solve:                        optimal
    Status crossover:                                   not run
    objective value:                                    6.65746563e+09
    interior solution primal residual (abs/rel):        1.82e-11 / 1.74e-16
    interior solution dual residual (abs/rel):          2.52e-05 / 4.54e-11
    interior solution objective gap (abs/rel):          4.59e+05 / 6.90e-05
Ipx: IPM       optimal
WARNING: LP solver residuals: primal = 62.5; dual = 1470.48 yield num/max/sum primal (126580/62.5/561308) and dual (46575/1470.48/2413.75) corrections
WARNING: Unwelcome IPX status of Unknown: basis is not valid; solution is valid; run_crossover is "off"
Model name          : linopy-problem-hfebb0yy
Model status        : Unknown
IPM       iterations: 57
Objective value     :  5.2139443572e+09
Relative P-D gap    :  1.7754187089e+01
HiGHS run time      :        759.97
Writing the solution to /tmp/linopy-solve-q1omjfnn.sol
WARNING:linopy.solvers:Solution status unknown. Trying to parse solution.
WARNING:linopy.constants:Optimization failed: 
Status: unknown
Termination condition: unknown
Solution: 344624 primals, 890676 duals
Objective: 5.21e+09
Solver model: available
Solver message: unknown

WARNING:__main__:Solving status 'unknown' with termination condition 'unknown'

Steps To Reproduce

Im just running a small model with the following differences from the default config.

scenario:
  interconnect: [western] #"usa|texas|western|eastern"
  clusters: [4m]
  simpl: [12]
  opts: [REM-12h]
  ll: [v1.0]
  sector: "G" # G
  planning_horizons: [2030]    #(2018-2023, 2030, 2040, 2050)
foresight:  'perfect' # myopic, perfect

model_topology:
  transmission_network: 'reeds' # [reeds, tamu]
  topological_boundaries: 'reeds_zone' # [county, reeds_zone, state]
  interface_transmission_limits: false
  include: # mixed zone types not supported
    # reeds_zone: []
    reeds_state: ['CA']
    # reeds_ba: []
  aggregate: # eligible keys: [reeds_zone, trans_reg]
    # trans_grp: []
    # reeds_zone: []

Expected Behavior

No response

Error Message

Anything else?

Not sure if this relates to solution quality as mentioned in issue #633

Reproduce the issue using the listed scenario configuration and the environment described in envs/environment.yaml, comparing the Gurobi, CPLEX, and HiGHS logs. Start by checking how the solver result is classified when HiGHS reports an unknown status and when CPLEX cannot save a solution. Done means the cause is identified and non-Gurobi solver runs either return a valid optimization result or a clear actionable failure.