1) How can I give '<h2>Related Topics</h2>' a h2 class="X"?
2) Is there a way to display nothing, when no related posts are found? So the whole div will be hidden?

Cheers _ck_

You'd go into
function related_topics_get_links($list) {

The line that does the output is

$links[$topic->topic_id]="<a href='".attribute_escape($link)."'>".wp_specialchars( $topic->topic_title, ENT_QUOTES)."</a>";

But you'd also need the data, so first change this line

$query="SELECT topic_id,topic_slug,topic_title,topic_posts FROM $bbdb->topics $where LIMIT ".$related_topics['maximum_items'];

Then you can change this like so:

$links[$topic->topic_id]="<a href='".attribute_escape($link)."'>".wp_specialchars( $topic->topic_title." ($topic->topic_posts)", ENT_QUOTES)."</a>";

Actually, by further testing it, one can
a) get no error and have a nice list of related topics;
b) get the error and also a related topics list;
c) get just the error.

It all seems to be dependent fron the "keywords" used.
I do not know *why* the error is generated, but as one obviously cannot control the keywords, I'm afraid I have to deactivate this pugin.
Pity, for it is really nice!

Do you have phpmyadmin? Maybe try that query directly and see if it also gives an error, or vary the terms ('crema' and 'catalana' ) and see what happens

bbPress database error: [You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT 'crema' AS wd UNION SELECT 'catalana' ) AS v1 ON s1.top]
SELECT topic_id, COUNT(*) as count FROM bb_topics AS s1 JOIN (SELECT 'crema' AS wd UNION SELECT 'catalana' ) AS v1 ON s1.topic_slug LIKE CONCAT('%',v1.wd,'%') GROUP BY s1.topic_id HAVING count>=1 ORDER BY count DESC LIMIT 100

> You get that on every page load?

No. Just on the initial load. As I reload the page no error is shown.

> Do related topics actually come up?

Never.

> bbpress 0.9 or 1.0?

0.9.0.4

> mysql 4 or 5?

Mysql 5 on Linux/Apache

What do you think, please?

bbPress database error: [You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT 'normalized' AS wd UNION SELECT 'material' UNION SELEC]
SELECT topic_id, COUNT(*) as count FROM bbpressforumtopics AS s1 JOIN (SELECT 'normalized' AS wd UNION SELECT 'material' UNION SELECT 'functions' ) AS v1 ON s1.topic_slug LIKE CONCAT('%',v1.wd,'%') GROUP BY s1.topic_id HAVING count>=1 ORDER BY count DESC LIMIT 100

i guess that it is a problem of related topics plugin? Thanks for any help or people with same problems!

There are several huge lists of stop words available however I wanted to keep the performance reasonable so I only used a smaller list by default.

When tags were "invented" (or at least brought to the mainstream) I didn't like them but seeing them match actual topic titles is far more useful.

On a big forum I always wondered what might be a good incentive to get members to add quality tags, but it's hard to find a way. I wonder if the Amazon Mechanical Turk could be put to use by having several strangers look at the title and first post and seeing what keywords they come up with - those that come up with the same, there's your tags. Would costs a few pennies a topic though, that might add up fast.

I like the "stop word" feature too. Words like "without" were driving Related Post matches, and I thought maybe the matching algorithm wasn't working. But of course it was working great; I just needed to add some basic stop words. Very handy :-).

One word of warning: I am now somewhat addicted to checking the Related Posts for every topic on my boards. I click in to see if they are good matches, and then add tags until the Related Posts improve. If you have an addictive personality, you too may get sucked into this twisted habit!

I'll put it down on the "to do" list ;-)