After I took ED-E to the Followers for the weapons upgrade and 2 days later fetched him from Primm, it seems to have started glitching and not attacking at all anymore. I mean it hasn't fired one shot after the upgrade, and on top of that it seems to get randomly stuck (I hear the buzzing though, meaning that it's trying to move) and the only way to temporarily fix that is to fast travel anywhere.

Could this be related to the fact that I have a mod that makes companions essential (instead of dying, they fall unconscious)?

Followers in FONV are already essential and can't die? Why you'd have that mod I have no idea.

Not with hardcoe mode they aren't.

I'm not using hardcoe and i don't have any mods and the same thing happened to me... with both endings to the ED-E quest

Same happens to me, once combat, ED just grind to a halt, and wont engage into combat, or start move again afterwards. Can fast travel and he moves with me, and ressume move around again, until combat again, then stops... Annoying.

Exactly. I think there's a bug when it tries to start attacking which glitches it, and only resets on fast travel.

While I did not experience this yet, it should be known that

*spoiler* before you give up ED-E to be upgraded, MAKE SURE TO TAKE ALL ITEMS OFF OF IT!! I did not think to do this, and when it was returned to me, it was empty; I had the . . . Q35 (?) energy weapon amongst other things on it. *end spoiler*

So now I have to lose about an hour or so to get to a previous save. My own fault, but still sux.

Try opening the console, clicking ED-E and writing:

additem 001694DF 1

If you have ED-E with upgraded weapons or

additem 00166B94 1

If you have ED-E without upgrade or with upgraded armor. Now open the companion wheel and tell him to switch to melee weapon and then back to ranged and it should work (untill it bugs again). The sound from firing his laser might be not be there untill you fast travel somewhere else.

From what I have seen the freezing bugs happen because ED-E doesnt have his ranged weapon in his inventory which leads to his AI freezing when it tries to shoot things. Especially entering Gomorrah in my game seems to strip ED-E and whichever follower I have with me of their weapons and not getting them back when leaving the place like they should.

Yeah, that fixed it. Thanks!