SharePoint Crawl Errors and Resolution

ERROR No. 01
Failed to retrieve data from the External System: 'MethodInstance with Name 'AttachmentStream' on Entity (External Content Type) with Name 'SFDCArticle_CEntity' in Namespace '.BCS.Connector.SFDCArticle_C' failed unexpectedly. The failure occurred in method 'GetAttachments' defined in class 'SFDCArticle_CService' with the message 'Sequence contains no elements'.

Solution:-

When you get the LINQ Error "Sequence contains no elements", this is usually because you are using the First() or Single() command rather than FirstOrDefault() and SingleOrDefault().
https://stackoverflow.com/questions/1324199/sequence-contains-no-elements

ERROR No. 02
Failed to retrieve data from the External System: 'MethodInstance with Name 'ReadItem' on Entity (External Content Type) with Name 'SFDCArticle_CEntity' in Namespace '.BCS.Connector.SFDCArticle_C' failed unexpectedly. The failure occurred in method 'ReadItem' defined in class '.BCS.Connector.SFDCArticle_C.SFDCArticle_CService' with the message 'Object reference not set to an instance of an object.'.'

Solution:-
Empty File OR Recrawl

ERROR No. 03