This will be fixed in the next release.

The next release will fix this.

This error is coming from the AFX plugin and contains no useful information whatsoever.

There are many limits on what geometry you can have in a BSP map file, and Blender doesn't know how to adhere to any of them. An importer might make sense but an exporter would be too hard to use.

On top of that, Source 2 has a completely different format.

Colours changed in 2.80 and now have four components. I'll get this fixed.

Vertex colours are not supported by the SMD format.

I have no idea what you changed, or why. I will need at least an example blend file. It would also be very helpful if you applied your changes to a clone of the source tools repo, so that a diff can be made between them.

Do you have Blender's debug mode turned on? You can check with the Debug operator.

Someone else reported this. It's an issue with the model having a second modifier, as yours does. If you apply/disable/remove it the problem should go away.

I'm not sure why this is happening and won't be able to look into it in detail for some time.

I tracked it down to the edge split modifier on the model. Applying/removing/disabling that fixes the issue.

I'm not in a position to debug this properly so there won't be a fix for a while. My instinct is that it's a bug in Blender 2.80, as modifiers should be transparent to export scripts.