Check Basic Antispam plugin settings.
You can also search for individual words your users use in Antispam blacklist on Tools > Antispam tab.

Basic Antispam wasn't enabled.

One reader kindly emailed the comment that couldn't be posted. After some trial and error, i found out it's the word 'suggested', which isn't in the blacklist! I tried to comment with just this one word and got the illegal content message. But if I comment as admin, it went through.

Any idea?

There might be only part of this word in the blacklist.
Search for combination of 5 consecutive symbols
sugge
ugges
ggest
geste
ested

You can also backup and clean up antispam table.

Finally have time to have a look at it.
Yes, one of those consecutive letters... it's not even a word! not sure how it ended up there. Thanks for pointing it out.