Reports: date not visible in "Recent log entries"

When I run the report "Recent log entries" (admin/reports/dblog), the "Date" column contains no data - it's just empty.

I can still re-order the report table using the "Date" column header, and if I click on a link in the "Message" column the date and time are shown in the "Date" field. The MySQL table "watchdog" contains data in the "timestamp" field for all records. The report just does not display the date.

I upgraded from Drupal 6.6 to Drupal 6.9, but this did not help.

I restored the MySQL database from a 6-week old backup and the "Date" column in "Recent log entries" correctly displayed dates for every entry. This suggests to me that the problem lies in the MySQL database.

I restored the current database and the problem recurred. I disabled all the non-core modules and uninstalled all non-core modules that could be uninstalled, but that did not help.

I emptied the MySQL "watchdog" and cache tables, but that did not help.

I used phpMyAdmin to check all the MySQL tables which reported some problems with INDEX keys (see below). Repairing the tables does not seem to fix these.

Problems with indexes of table `blocks`
UNIQUE and INDEX keys should not both be set for column `theme`
Problems with indexes of table `filters`
UNIQUE and INDEX keys should not both be set for column `format`
Problems with indexes of table `menu_links`
More than one INDEX key was created for column `menu_name`
Problems with indexes of table `node`
PRIMARY and INDEX keys should not both be set for column `nid`
Problems with indexes of table `system`
More than one INDEX key was created for column `type`
Problems with indexes of table `term_data`
More than one INDEX key was created for column `vid`
Problems with indexes of table `term_hierarchy`
PRIMARY and INDEX keys should not both be set for column `tid`
Problems with indexes of table `view_view`
UNIQUE and INDEX keys should not both be set for column `name`

I'm not sure what to do now. Any suggestions would be welcome!

Regards,
Chris.