I'm still not quite sure what causes this, but this happens most often when updating a packaged asset or replacing another asset with the same name.
What is strange about how the FB handles packaged assets is that they are automatically copied (in the background) to the bin-debug folder. this copying occurs no later than when you create / debug, but you can rejoice almost immediately as soon as you drag the asset into a folder identified as a resource folder (that is: it already contains some packed assets).
The solution for me was to open bin-debug and completely remove it from all packaged assets. Then open the "Properties" in the project and go to the settings "Packaging", Reset to "Default", and then go in and check / uncheck the boxes with the assets that you want to pack. Apply the changes and wait for the files to be copied. Then try assembling / debugging again, and usually everything works.
The bonus aspect of this is that your bin-debug folder is cleared of any fissured that can be accumulated from older, now no longer used, assets. they simply clutter up your package settings and in the worst case, they can end up in the final packaged product, unnecessarily inflating it.
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