Blank Comments

I get a few moderation emails per day asking me to approve comments on older blog posts, but the comments look like this:

Author : (IP: , )
E-mail :
URL :
Whois : https://whois.arin.net/rest/ip/
Comment:

I’ve already tried all of the following:
– deactivating plug-ins.
– reinstalling WordPress core files.
– using the default TwentyTen theme.
– disabling comments on blogs older than 14 days.
– the database is fine, no security threats with any scripts.

Any help/advice will be appreciated!