<Ivor>
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PS
I assumed that since the bug occurs when a mod is connected, it means the mod is influencing it, and in that case, I would have to patch the mod itself, possibly its DLLs and not the executable. That's why I wrote here.
You need to patch the executable, ignoring the HD version. Then, you can see if there are any bugs. You need to find the code where the ground for the underground battle is set and substitute the correct type. The current implementation is a potential source of bugs, so it's better to do it properly right away, rather than waiting for changes in the HD version.