Infeasible model - Free variables

[Timon-R]

Checklist

• I am using the current master branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

I run the up-to-date default model with 39 nodes and 6H time resolution and get an infeasible model. The only difference from the default environment is that I use gurobipy==11.0 as I don't yet have a license for 12.

Error Message

This is how my output looks like (all the previous rules functioned just fine):

Barrier solved model in 0 iterations and 0.54 seconds (0.72 work units)
Model is infeasible or unbounded
INFO:linopy.solvers:Unable to save solution file. Raised error: Unable to retrieve attribute 'X'
WARNING:linopy.constants:Optimization failed: 
Status: warning
Termination condition: infeasible_or_unbounded
Solution: 0 primals, 0 duals
Objective: nan
Solver model: available
Solver message: 4

WARNING:__main__:Solving status 'warning' with termination condition 'infeasible_or_unbounded'
Gurobi Optimizer version 11.0.0 build v11.0.0rc2 (mac64[arm] - Darwin 24.2.0 24C101)

CPU model: Apple M2 Pro
Thread count: 12 physical cores, 12 logical processors, using up to 8 threads

Iteration    Objective       Primal Inf.    Dual Inf.      Time
       0      handle free variables                          1s
  428473      handle free variables                          5s
  627213      handle free variables                         11s
  726807      handle free variables                         17s
  777836      handle free variables                         20s
  881362      handle free variables                         27s
  988311      handle free variables                         33s
 1047481      handle free variables                         36s
 1165066      handle free variables                         43s
 1226119      handle free variables                         46s
 1361958      handle free variables                         53s
 1433740      handle free variables                         56s
 1585794      handle free variables                         63s
 1664060      handle free variables                         66s
 1820592      handle free variables                         72s
 1898858      handle free variables                         75s
 2055390      handle free variables                         81s
 2211922      handle free variables                         87s
 2368454      handle free variables                         92s
 2524986      handle free variables                         97s
 2681518      handle free variables                        100s
 3057940      handle free variables                        109s
 3136206      handle free variables                        113s
 3449270      handle free variables                        116s

Restart IIS computation due to numeric trouble ...
Iteration    Objective       Primal Inf.    Dual Inf.      Time
       0      handle free variables                        119s
  199800      handle free variables                        120s
  548947      handle free variables                        125s
  676719      handle free variables                        131s
  777836      handle free variables                        138s
  829581      handle free variables                        141s
  934012      handle free variables                        147s
  988311      handle free variables                        150s
 1105937      handle free variables                        157s
 1226119      handle free variables                        163s
 1293501      handle free variables                        166s
 1433740      handle free variables                        173s
 1508910      handle free variables                        177s
 1585794      handle free variables                        180s
 1742326      handle free variables                        187s
 1898858      handle free variables                        193s
 1977124      handle free variables                        195s
 2133656      handle free variables                        201s
 2290188      handle free variables                        207s
 2446720      handle free variables                        212s
 2603252      handle free variables                        216s
 2838050      handle free variables                        220s
 3057940      handle free variables                        226s
 3136206      handle free variables                        230s
 3558031    0.0000000e+00   5.399926e+09   0.000000e+00    236s
 3618981    0.0000000e+00   5.496348e+09   0.000000e+00    254s