Subscribe to comments : weird behaviour

Hello peeps
I’ve installed succesfully Subscribe to Comments By Mail by Scriptygoddess. What is less succesful is that I get random errors : sometimes it’s ok, but sometimes I get the message “Error: please type a comment” on wp-comments-post.php (but the comment has been posted anyway)

Any idea of where I should look ? It seems to be a weird server configuration I have to deal with …