I'm having a strange problem combining objects that look to be paths as far as I can tell. 

I successfully selected all items and converted Strokes To Paths via the Path menu. 

I then selected all parts and ungrouped. When selecting everything, the Ungroup is not an option in the right click mouse menu. That should mean nothing is in a group. 

When I try to Union via the menu or the keyboard shortcut, Union fails. There is no message why it fails, the status bar doesn't change. Nothing happens. 

Union only works by selecting each path individually. I cannot drag a box, selecting parts and then apply Union. 

I looked at the selection setting in Preferences and did not see anything obvious that might correct this. 

Is this an undocumented feature of selection? I cannot figure out why dragging all the strokes and applying Stroke to Path works, while selecting all the paths, Union does not work. 

Example file attached. 

I would open object's panel to be sure to check if there are no groups left.

Edit : took a look at the svg file : path299 and path63 are weird paths (their d attribute in xml editor seems broken). 

If you deselect or remove them, it works. 

Seems like a bug.

Those zero length paths should render as dots like they do in browsers.

If you replace Z with v0 they show up in Inkscape.