Hi there,
This is interesting, I'm no expert when it comes to dream interpretation but I do try from time to time!
This dream is funny though, if it weren't for you stating you have no feelings left for your ex boyfriend I would say...you still have feelings for him! So here, I'm inclined to point out the opposite, that maybe he still has feelings for you and that subconsciously you are aware of this. Have you noticed your friends seem to be watching your every move and chatting about your "friendship" more than you feel necessary?! If you were once in love and are now able to be friends, perhaps there's still room for extra feelings besides those you have for other normal friends. Plus the time you've spent away from eachother might have been just what he needed to settle and realise how happy he was when you were together.
Look at this info on the "basement"
To dream that you are in a basement, symbolizes your unconscious mind and intuition. The appearance of the basement is an indication of your unconscious state of mind and level of satisfaction. It represents primal urges, animalistic desires and basic needs. Maybe you're on the look out to be as happy as you felt that night, but be wary of your ex/friend, he might not be the one to help you with this. By the sounds of it you have probably moved on. If you can talk to him about this, you might be able to clear up any confusion and continue to be good friends

Lots of luck,
Linzi