Postgres copy data with \xYY as plain string instead of interpreting as encoded string
The backslash \ is the default escape character in the (default) text format of COPY. The format you describe is recognized as (quoting the manual here) 


  backslash sequences are recognized by COPY FROM
  ...
  \xdigits | Backslash x followed by one or two hex digits specifies the character with that numeric code


.. which seems to be just as intended.
To avoid this effect you could use the CSV format instead, where the backslash has no special meaning:

COPY tbl FROM '/path/to/file/log.sql' (FORMAT csv);


Be sure to read the manual to learn about additional differences.

Alternatively you could replace every \ with \\ in your file with a sed script or something.

If the above doesn't solve your problem, please supply:


PostgreSQL version
Database encoding
Demo data
Definition of the table you COPY to
The exact COPY invocation you used