So far, the problem in the topic title has not manifested for a whole week
On a side note, the router restared itself twice last night while watching some tv show episodes (the files were smaller than 1GB). I've streamed mkv files larger than 30GB and all went fine, so I don't think it's a problem with the file sizes.
Last thing, I don't know if this is related to SmartConnect (which is ON), but often when I go into different rooms in the house, the devices lose connectivity (phone and laptop) and even if they see the wifi network, they do not connect automatically. I click on the network and then connect, they don't do anything. On the second or third time they connect and stay connected.
I've received an RMA approval from the shop (with the help of the people at d-link which have helped me in this topic) and tomorrow they will be returning the item to warranty. I'll post an update afterwards.