What happen with the Log backup if I set the SQL Server clock back? What happen with the Log backups if I set the SQL Server clock back? In countries with daylight saving this happens at least once a year. Will the point in time restore with the STOPAT option fail? 

Using fn_dump_dblog I see this after setting the clock back two hours.
A point-in-time restore to an a timestamp that appears twice in the log file will either fail, or arbitrarily select one occurrence of the target time.  I don't know which, and it doesn't really matter. You can restore to LSN instead of a timestamp using STOPMARK instead of STOPAT.