Lucene on windows server 2012

We've migrated 2 completely different sites that run Umbraco (6.1.5 and 6.1.2) to a new dedicated server that runs Windows Server 2012. We're having a very weird behavior when we start publishing new contents. After some time (but not always) the site simply stops working with this error: Http Error 500.19 The requested page cannot be accessed because the related configuration data for the page is invalid. Error code 0x80070006

This is happening on both sites but one doesn't affect the other. Only an application pool recycle fixes it. If we disable all the indexes this never occurs. Everything was ok on the previous servers.

Anyone else had this problem or has any idea of why this is happening?