i've got a form, authored inside a page template:

<form enctype="multipart/form-data" method="post" action="merci">`
[... some inputs of different types: text, password, file ...]

<input type="checkbox" name="themes" value="$taxonomy_term_id"/> 
<label for="themes" > $term_name </label>

<input type="checkbox" name="themes" value="$taxonomy_term_id2"/> 
<label for="themes" > $term_name2 </label>

[...]

<input type="submit" />
</form>

The shape is working good, however when i check some checkbox, after posting, i acquired error 404 page not found ( the experience page "merci" ). If no checkbox are checked, all work and that i redirect towards the "merci" page.

Any clue?

<input type="checkbox" name="themes" value="$taxonomy_term_id"/> 
<label for="themes" > $term_name </label>

<input type="checkbox" name="themes" value="$taxonomy_term_id2"/> 
<label for="themes" > $term_name2 </label>

You've two checkboxes with similar name= attribute. Provide them with unique names.