It's only this location or does that also happen in other locations where the building is?

Thank you for the Christmas present! Good tidings to you, and your kin.
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Found the issue, will be fixed in the patch.

Note: although I have received help from the Discord on uploading images and files (it's drag and drop; no buttons... wish I could create a feedback ticket on feedback tickets..), I unfortunately still can't play this mission.

PS I have a screenshot and an error report. How do I attach these to this bug report?

We need more information. For example, what steam button?

Ok the geometry was modified, should prevent players from clipping inside.

Tried it again and recorded it. You can more consistently get in by vaulting and then moving backwards when your camera is on the other side of the wall. You might have to vault again.

I cannot replicate this, geometry works correctly for me.

Looks like its the top cover that's causing this, probably some strange rendering error with arma that casts shadows only for that specific bit and not the rest of the top cover, not sure this can be resolved.

The soldier class requires the Marksman DLC, which is why we didn't add it. Players might be confused to see the WS icon next to it but then seeing Marksman DLC advertisement (if they don't own it).