Are you talking about javascript alert message "Please do not send empty comments"?

I can't confirm the bug in [url=http://test.b2evo.net/HEAD/blogs/admin.php?login=admin&pwd=demopass]Test server[/url]

What PHP version do you have? Is mbstring extension loaded?

No, I am talking about the PHP error page which is with a white page.

My PHP version is 5.2.14 and mbstring is loaded.

I was able to reproduce it on the test server.

Steps to reproduce:

1. Go to Control panel -> Global settings -> Plugins.
2. Install Basic antispam plugin if not installed.
3. Edit settings and enable Remove repetitive characters.
4. Try to post comments with multiple lines for example in here:
http://test.b2evo.net/HEAD/blogs/blog1.php/welcome-to-b2evolution#feedbacks

It randomly gives the error message that do not post empty comments.

I haven't yet figured out what causes the error in the message. The problem is that the error message is very random and I had to post several comments both on my own blog and in the test server before seeing it.

But many our users reported seeing the error when the setting was enabled.

Yep I saw the same thing also. I can confirm this. Had to disable basic antispam to clean it up :(

(Alex/sam2kb, this is your feature, I leave it up to you to find out what's going on.)

Well if there's a problem it must be in this regexp, and I really don't see a case when it can output an empty string or error

preg_replace( '~(.)\\1{3,}~u', '$1$1$1', $str );

Ok, I sort of fixed this in CVS.
The function now returns the original string if some error occurred during character substitution.