I've downloaded your datasets and discovered that your cable.TAB file contains multipart features. This alone should not prevent the copying and pasting of features into another dataset. However, trying to paste the features caused QGIS to not respond. I suspected there was something about the geometry that wasn't valid.
In fact, when I looked more closely at the direction of the lines, the segments of one of the multipart lines were going in opposite directions. This may have been the reason you can't paste them.
I was able to explode the features into single part features using the following method in QGIS 3.0.3:
- Select all the features in cable.TAB
- In the Vector Menu, Geometry Tools, Multipart to singleparts
- Copy all single part lines
- Ensure cable2.TAB is editable and selected
- Paste all features into cable2.TAB
You could also inspect the lines individually, explode them, reverse the ones going in the wrong order, and connect them back up again.
Generally, for things like this, I think unless you have a very good reason to have multipart lines, I suggest you convert to single parts. You'll run into less problems like that.