There is an interesting paper (the main author seems to be from Netherlands, too) about potential mechanism of action that makes Covid infections have long-term repercussions published last year at https://www.frontiersin.org/journals/immunology/articles/10.....

> Based on these data, we hypothesize that the shift to anaerobic respiration causes an acid-base disruption that can affect every organ system and underpins the symptoms of PASC.

(Brain, of course, is also an organ, so brain fog and reduction in mental capabilities could easily fall under this.)