Your add to cart button should look like this:
<button type="button" title="Add to Cart" class="button btn-cart" onclick="productAddToCartForm.submit(this)"> <span> <span><div class="cart-text">Add to Cart</div></span> </span> </button>
You should check your xml and phtml file associated with add to cart function.
For bundle product you can start checking on layout/bundle.xml and also in layout/catalog.xml.
I would suggest to look more specifically in
<block type="catalog/product_view" name="product.info.addtocart" as="addtocart" template="catalog/product/view/addtocart.phtml"/>
This above block is called from bundle.xml file which looks like this:
<block type="bundle/catalog_product_view" name="bundle.summary" as="bundleSummary" template="bundle/catalog/product/view/summary.phtml" translate="label">
<label>Bundle Product View Summary</label>
<block type="bundle/catalog_product_price" name="bundle.prices" as="bundle_prices" template="bundle/catalog/product/view/price.phtml">
<action method="setMAPTemplate"><tmpl>catalog/product/price_msrp_item.phtml</tmpl></action>
</block>
///// here it is called
<action method="append"><block>product.info.addtocart</block></action>
<action method="append"><block>product.info.addto</block></action>
</block>
I am sure your custom theme is playing the role.
If you still cannot find the solution, then you can try activating default theme and see if problem actually with your theme.
Good luck.
Best Answer
If you are not sure where to find the web server logs (the error details are there), open index.php file and uncomment the following line
So, you will have in result
Save the file and try the add to cart operation once again. You should get the error details on your screen. Do not forget to comment the mentioned line once you have the error details, especially on the production environment.