Что заставляет xcopy говорить мне Доступ запрещен?

подходит ли вам это?

set @gr=1;
select uid, max(cnt) max_cnt from (
  select uid, grp, count(*) cnt from (
    select uid, 
      case when ifnull(DATE_ADD(oldDate, INTERVAL 1 DAY), timestamp)= timestamp then 
        @gr 
      else  
        @gr := @gr +1 
      end grp
    from
    (
        SELECT    
          uid, 
          timestamp, 
          lag(timestamp) over (partition by uid order by timestamp asc) as oldDate
        FROM      OnSite
    ) t
  )t2
  group by uid, grp
  )t3
group by uid

Результат

| uid    | max_cnt |
| ------ | ------- |
| u1     | 2       |
| u12345 | 4       |

DB Fiddle