I believe that the upgrade from 2.x to 2.5 did not unset the “gzipcompression” option in the wp_options table. If you’re comfortable with modifying the mysql database (and have the permission to do so), the following should help:
update wp_options set option_value=0 where option_name="gzipcompression";
You can then verify it by doing:
select * from wp_options where option_name="gzipcompression";
Which should result in something like:
+-----------+---------+-----------------+--------------+----------+
| option_id | blog_id | option_name | option_value | autoload |
+-----------+---------+-----------------+--------------+----------+
| 35 | 0 | gzipcompression | 0 | yes |
+-----------+---------+-----------------+--------------+----------+
1 row in set (0.00 sec)
