I encountered the following error while crawling our site collections.
Processing this item failed because of an unknown error when trying to parse its contents. (Error parsing document ‘<doc>’, Sandbox worker pool is closed ; ; SearchID = <GUID>
Only titles of document were crawled using search which resulted in search only showing titles and Office Web Apps preview wasn’t working.
I have granted the service account the proper permissions and configured the service account correctly using Central Administration –> Security –> Configure Service Accounts
You can first try to use the farm account because this service account should have the appropriate permissions. The errors were gone after a full crawl of all content sources and search was working again.