ORA-12546: TNS:permission denied, on Windows through Task Scheduler
The problem lies somewhere deep in windows permissions. Those are messing somehow with Oracle TNS system. I still do not know the right way and what exactly those permissions are, but the problem is solved for now, by means of bypassing TNS, as self-suggested previously: 

set ORACLE_SID=dbname
exp.exe myuser/pass owner=myuser log=mylog.txt

This works, no error