Your rashes must be incredibly uncomfortable and distressing. Just to put in my experience with ALA:
Have been taking for over 12 yrs or so now, upon recommendation from an incredible rheumatologist who understood my not wanting a prescribed medicine (my neurologist wanted me to take gabapentin) for the neuropathic situation in my feet and hands. The ALA "kicked in" and the issues with neuropathy in feet (especially the toes) and in hands dissipated. Not totally gone, but made life so much more tolerable. My bloodwork did not show diabetes or other factor(s) that would explain the "why" of the neuropathy. During all these years, no rashes attributable to ALA. Same for my husband, who had been taken ALA for a few years before I commenced including it daily.
I hope you obtain an answer to your mystery itching/rash.