The anti-spam list is not yet checking for words, but is checking for URL's.
Are you using a URL that is on the anti-spam list for some reason ?

No, I'm not using any links, just words.
Could signs such as ¿ "" affect?

It seems the problem is accented characters.
I can't publish accented characters in comment viw (logged in) but when I edit the comment I can add the accents.

Anybody knows how to solve this?

What version are you using? Does it also happen on the demo with Phoenix?

I'm using Dawn with es-VE (spanish, Venezuela).

So I guess(!) it happens also on the demo. can you please paste what causes the error? Because I can comment with

No, I'm not using any links, just words. 
Could signs such as ¿ "" affect?

just fine. Also accented characters as "éè".

Any of the following characters are not accepted: á é í ó ú ñ

What I have to do in able to post my comments with accents is:

Log IN
Write comment without accents, then publish.
Then I "Edit" the comment, add the accents and publish. On "Edit" mode accents are accepted.

The last time I tried to post with accents, I could't until I deleted all accented characters and posted the rest of the text.
But I didn't do something obvious that is try to publish just accented characters... I did it now and found that there is no problem with the accents. So the problem is a word that has an accent but I don't know which one it is.

Is there a way of knowing which word is being banned?

Sorry about making you waste your time but I'm knew to b2evolution and there are too many things I ignore.

Does anybody know how to detect which words, URLs, whatever are getting banned from comments?
I'm getting some mails about people trying to comment on my blog but getting a message saying "supplied comment is invalid".
Yet when they mail me the message I don't see any word, url or anything that is in the anti-spam list (apparentely).
Can anybody tell me if there is a way to findout what in a comment is being banned?
This is urgent since this problem is very annoying to my visitors...

Thanks

I found the answer to my own question and it is something that should be addressed by the b2evolution designers.

The answer can be found here:
http://forums.b2evolution.net/viewtopic.php?t=5820

And the problem that is happening with my blog is that words are been banned, not just Urls.
Worst yet the words that are being banned are part of a banned url. To make this point clearer I'll give an example.

The spanish word "Tambien" (which means "also") is being banned because of the banned term "ambien". That means nobody can write "also" (in spanish) in my blog or else they will be banned.
I suppose that is happening with many other words.

Now, how can that be avoided? This is a big problem!

You're running 1.6 Alpha? That is the only version that checks words in comments. Anyway unban "ambien" and locally ban punctuated ambiens: "-ambien" and ".ambien". It is highly unlikely that someone will write "T-ambien" or "T.ambien" eh? Ideally banned keywords will have punctuation before and after, or at least look like a domain name. In the case of ambien (and many other drugs) the singular word-like word has been an extremely effective banning tool. It's a change in 1.6 that results in the text string matches blocking reasonable comments.

I'm running Dawn. I did what you suggested and it worked with "Tambien". What was strange too is that "tambien", properly written, has an eccented e (é) and written that way was also getting banned.

I have the same problem on 1.8.2, the users are being tired of try to post comments... :'(