I was able to locate my problem of brain fog/concentration accurately. It went away with large doses of B12 in the order of 0.5-1 mg.
However, when the brain fog went away I also got hyperthyroid symptoms. The brain fog would return after discontinuing the use of B12, and hyperthyroid symptoms would also go away.

My concentration and brain fog problems went permanently away with ALA chelation, and I could feel a movement within the thyroid, it probably shrank in size, because swallowing is now easier. I never got a diagnosis, because this didn't show up in bloodtests.