Thank you for testing out the Release Candidate.

You can attach a (.NET) debugger to the running Python process and see where exactly the exception is raised, without a stack trace or a reproducible example of some sort we won't be able to help you.

I should have noted, everything I'm doing is from python and I'm working in VS Code. .NET is not my specialty, so I'll need some help on that debugging step. I took a look through some of the documentation on the wiki and it didn't look like it was relevant, requiring Visual Studio proper. Can you help point me in the right direction?

I have never tried to debug (apart from classic print debugging) outside of Visual Studio. The Community edition is free, so I suggest you try debugging with that.

I'd really like to fix this one before releasing the final 3.0, but there is really not much to go on here. Can you create a reproducible example for this case?

I spent a little time working to get debugging set up with Visual Studio yesterday and didn't have much success getting the debugger attached. With little success, I approached the problem from a slightly different direction... I noted in the OP that I have a module set up to act as a wrapper around the .NET functionality to try and create a clear delineation between Python and .NET. Rather than use the wrapper, I opted to use the "regular" code:

import os
import sys
from pathlib import Path
import pandas as pd

# Add to PATH
dllPath = Path(r'C:\Program Files\Energy Exemplar\PLEXOS 9.0 API')
sys.path.append(str(dllPath))

import clr
clr.setPreload(True)

clr.AddReference('EEUTILITY')
import EEUTILITY.Enums  # type: ignore

clr.AddReference('PLEXOS_NET.Core')
import PLEXOS_NET.Core as PLEXOS  # type: ignore

clr.AddReference('EnergyExemplar.PLEXOS.Utility')
import EnergyExemplar.PLEXOS.Utility.Enums  # type: ignore

sol = PLEXOS.Solution()
sol.Connection("Model Test Solution.zip")

sol.QueryToCSV(
    'test.csv',
    False,
    EEUTILITY.Enums.SimulationPhaseEnum.STSchedule,
    EEUTILITY.Enums.CollectionEnum.SystemRegions,
    '',
    '',
    EnergyExemplar.PLEXOS.Utility.Enums.PeriodEnum.FiscalYear,
    EnergyExemplar.PLEXOS.Utility.Enums.SeriesTypeEnum.Statistics,
    '29',  # USE,
    None,
    None,
    '',
    '',
    '',
    EnergyExemplar.PLEXOS.Utility.Enums.AggregationEnum.All,
    '',
    ',',
)

sol.Close()

There are a couple other ways to pull data out of the file, those all seem to work both through the wrapper and with snippets similar to the above. While it seems like there is some change being introduced between 3.0.0a2 and 3.0.0rc4 I'm also not convinced it isn't due to something funky with the wrapper and class inheritance.

Given that other methods seem to work, and that the error seems to be isolated to code that I wrote, I am comfortable shelving this and taking a little more time to review things on my side.

I'll close this one for now, as there is nothing "actionable" from our side. I would suggest that you open a bug report on the vendor's Github and just reference me in it. Since their API is apparently .NET Core nowadays, they could really profit from having this work.