The bug is in the auctions plugin itself. Page handler should return true after page generation. Otherwise there's high risk of rendering 404 page below.

@Rodolfo Try adding "return true;" at line 285 in start.php

Thank you @Paweł Sroka. This also happens in another plugin

http://community.elgg.org/plugins/407901/1.8f/market-plugin

and decided adding:

return true;

in: Market>start.php

before the closing curly bracket of function market_page_handler($page) { at line 95.

@ Rodolfo

Doing that in the Market plugin results in the following error:

Parse error: syntax error, unexpected '}' in /home/website/public_html/Elgg/mod/market/start.php on line 96

I have also not seen that error occure with the market plugin.