When trying to perform a record lookup within Flow, I've found the lookup now crashing under certain circumstances.  I had it in a dynamic query, so it took me awhile to figure out it was because of the lookup crashing that the dynamic query wasn't returning any choices.
I setup a Flow to try to isolate the issue.  I first query a custom object 'interest__c' for a record based on record type and have it return its name and 'Interest_Type__c' (pick list field).  I then lookup based on finding 'RI' in the interest name, again returning name and type.  Both of these queries succeed and return records showing 'RI' as 'Interest_Type__c'.  When I query a 3rd time filtering by 'Interest_Type__c' = 'RI', it crashes with the following error message.
======================================
Error Occurred: This error occurred when the flow tried to look up records: An unexpected error occurred. Please include this ErrorId if you contact support: 1590006369-71166 (-548403183). You can look up ExceptionCode values in the SOAP API Developer Guide.
======================================
Here is a screen shot of the debug log from this 3-part test: