I'm on premium Beta LEP as well and had no issues. No steam account here but from what I read it also seems to affect some non-steam accounts.Well it seems LEP. But with only 1 out of 4 accounts failing to access Horizons my position is obviously skewed if not. And it is definitely not steam as I'm not accessing via that.
To narrow further, I'm, premium beta LEP, so it may just be that one