Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator

I got this error while working on a SharePoint 2010 - Business Connectivity Services Project. The full error read like:

Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator

It was so that there was mismatch between business data model and the entity defined therein the project.
The resolution:

MisMatch was there between the TypeDescriptors provided by you in the business data model entity's methods and the entity class file (.cs file).

Setting 'The Identifier represented by this TypeDescriptor' wherever applicable.

Also mark the identifier in the typedescriptor wherever it is encountered. Like you will be having two type descriptors customer_id and customer_name of which 

customer_id might be identifier so set it up like that.