qgis - Trouble clipping two vector layers (EPSG is the same in both)

I am trying to clip a layer but it gives empty output (No error message, just an empty layer)

The desired input layer is a hexagonal grid (148980 features) created in MMQGIS in EPSG: 32629.

The clipping layer is a layer that I created myself, by merging two line vectors and then using the tool Lines to Polygons, also in EPSG: 32629, which you can see below.

Both layers are quite large, since they represent a portion of the Exclusive Economic zone of Portugal (around 100 000km2)

I'm working in QGIS 2.18.16