Of course, he has not only one, but several free choice salt licks. Apparently, he doesn't get enough salt from them alone and I don't have a way to provide free choice loose salt, so added to his feed it is.
The pastures are what we call "biological" here - different grasses and wild herbs, but nothing poisonous to horses found yet. Although this problem has, so far, reoccurred in different pastures and two different barns, bloodwork might be a good idea, thanks!
I have come a long way, to surrender my shadow to the shadow of my horse.