If you take a look at your error call stack
#0 /home/public_html/domain.nl/includes/src/__default.php(27333): Mage::throwException('Ongeldig blokty...')
#1 /home/public_html/domain.nl/includes/src/__default.php(27275): Mage_Core_Model_Layout->_getBlockInstance('relatedslider/p...', Array)
#2 /home/public_html/domain.nl/includes/src/__default.php(27310): Mage_Core_Model_Layout->createBlock('relatedslider/p...', 'magentothem_rel...')
#3 /home/public_html/domain.nl/includes/src/__default.php(27077):
most of the files involved come from the includes/src/__default.php file. Magento only uses this file when it's running in "compiler" mode. Compiler mode is when Magento looks at all the module class files in app/code/*/*, and then makes a copy of those files in includes/src — it also combines many base classes in a single file, __default.php. This is done to increase performance in production systems.
So, step one would be to disable compiler mode, see if the error has gone away, and then re-compile your classes to catch whatever class was missed in the initial compilation. You can do this at
System -> Tools -> Compilation
Or via the shell script
$ php shell/compiler.php
If the problem does not go away when compilation mode is disabled, it looks like Magento is trying to instante a block (perhaps embedded in a CMS page) that doesn't exist.
Mage_Core_Model_Layout->createBlock('relatedslider/p...', 'magentothem_rel...')
Unfortunately, this block's full class alias (relatedslider/p...) is omitted. Figuring out the full block alias, and why Magento can't instantiate it (missing class, mistyped alias, etc.) will lead you to a solution for your problem.
you can try below one
<checkout_onepage_success translate="label">
<label>One Page Checkout Success</label>
<reference name="root">
<action method="setTemplate"><template>page/2columns-right.phtml</template></action>
</reference>
<reference name="content">
<block type="checkout/onepage_success" name="checkout.success" template="checkout/success.phtml"/>
<block type="catalog/product_list" name="product_list" template="catalog/product/list-limit.phtml">
<action method="setColumnCount"><count>4</count></action>
</block>
</reference>
</checkout_onepage_success>
Hope this will work
Best Answer
If possible, I would recommend just disabling the Mage_Rss module. See Marius' answer on this question for an example of how to disable core modules.
https://magento.stackexchange.com/a/2664/2241
Zookal has done some research on what modules can be disabled without problems. Of course, if they are not being used. Mage_Rss is on their list of safe modules to disable (see list at the bottom of the page).
http://zookal.github.io/magento-mock/etc/challenges.html