Fixed in 3.4.1. The Action name will now be used instead of "Legacy Slot".

Blender Menu > Window > Toggle System Console

You can also find the operator by searching in the search popup.

It's all available under GPL, as each file states right at the top.

Please upload your .blend file.

I suggest sticking to Blender 4.3 for now.

I'm telling you to use Blender to edit your scene. Actions are no longer exported, just slots.

Luckily for you I have found how to do it. First you will need to make multiple copies of your armature and assign one action to each. Then you make the "real" armature the active object and execute the "Merge Animation" operator in the Dope Sheet / Action Editor:

https://docs.blender.org/manual/en/4.4/editors/dope_sheet/modes/action.html#action-menu

Afterwards the active armature will have one action slot for each other selected armature, and you can delete the armature copies.

Is this something you see when loading a dmx file? Try exporting to an earlier version.

You need to convert your separate actions to slots on one single action assigned to your armature. Then you will get the same behaviour as before.

I'm not actually sure how to do that, but there must be a way...right?

I can export the data from your file without any error. That said, I can see why the exception is being raised.

I created a branch which will fix that particular error. Please try it out and tell me if you encounter any other issues:

https://github.com/Artfunkel/BlenderSourceTools/tree/scientist_mdl

The importer doesn't do anything with sharp edges. Any sharp edges you see are something Blender did by itself.