ORA-01741: illegal zero-length identifier

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This is another curious error message that i presume is rare to be seen twice in your life.

alter session set tracefile_identifier="";

ORA-01741: illegal zero-length identifier

Oracle accepts the double quotes as string delimiter, but not when there is nothing in between.
In fact it is perfectly possible to execute:

alter session set tracefile_identifier="xyz";

select value
from v$parameter
where name = 'tracefile_identifier';

VALUE
-----
xyz

In order to set the session parameter to a null value, single quotes must be used:

alter session set tracefile_identifier='';

Note also that ALTER SESSION, unlike ALTER SYSTEM, doesn't come with an ALTER...RESET... clause.

See message translations for ORA-01741 and search additional resources.