Just adding, you can do this glitch regardless of whether or not you have anything in your hands.

You're trying to mend items that are incompatible with the leather sewing kit. As far as I've noticed though, you are unable to mend the proper items with a leather sewing kit.

Adding this note, I've just logged back in today and I was able to open the tent.

This also happens to melee weapons. Happens randomly, the game just acts as if I am constantly spamming/holding down left click.