Update of last post: Taldavid, you probably don't have to reset your Helix LT. As Christoph from line 6 told me, that he tried the looper with the preset Essex 15 I also did so and there the looper just played fine and I thought YES, the reset did it but than I recreated my backup with my presets and there we go again: Looper only half as loud...
But the good news is, I've found the problem! As soon as you put a looper on one of your two signal chains, you have to make sure that the input of the other route (in my case it is the lower line in the display where I see all my effects in a row) has to have a "X" in the input. My preset before was on both routes on "Multi" and therefore the signal was kind of split => only half volume of the looper as a result...
Hope that helps and all the best, Walter