On many positions the rocket turret and heavy cannon turret won't fire. We're using the Space Exploration mod with Rampant Enemy mod. They are aiming but not fireing. Any ideas what the problem could be?

Depends what is happening.

Turrets are deliberately set to begin aiming at biters slightly out of their maximum range - this pretargeting makes them more responsive than stock turrets, since they open fire the moment any biter steps into their range. So if that is happening, it's normal.

If turrets are not firing when enemies are within their actual range, then that's a bug. Please check the ranges of the turrets and see if biters are actually within their firing cones - many of these turrets have a minimum range, limited firing arc, or both.

I've checked the range and the biters and their bases are within. It seems to be, that this problem occurs, if there are many enemies and bases in the fire range, but this is just speculation. If i rotate the turret to target a position with few enemies it starts to fire.

Ok it seems to be that, if there are enemy buildings more near to the turret then the minimum range allows, the turret targets this building but won't fire :/ Is it possible to skip these buildings and just target things within the range?

Oh, I get it. Turret pretargets the nest/worm that is out of range and stays fixed on it.

Fixing that would require to eliminate the pretargeting feature entirely. I'd like to keep it though...

Can you take out the offending nests by artillery or manually yourself? That would be the easiest solution.

At this point we hadn't artillery and our base was surrounded by a lot of biters :/ , so killing them manually wasn't possible at all. Is it possible to have some kind of an option to deactivate the pre-targeting for specific turrets?

I'll add the option to turn off pre-targeting with the upcoming Factorio 1.1 update. It should be out in a few days.

Thanks a lot for your help! :)

Update for 1.1 is out.

Would it be possible to make this a "map" setting instead of a startup setting? I'm 10's of hours into a game, and just ran in to this issue and can't turn off the autotargeting. you may want to disable this entirely, or at least make it very easy to do so.

Can't change that into map setting - it's a turret prototype parameter, not a runtime one.