Random errors making simple GET calls to a panel data source

Client is running iMIS 2017 20.2.65.9955. Set up the panel yesterday, tested application with no errors. This morning the GET to the endpoint is throwing 500 errors - confirmed via Postman as well. Have tried recycling the app pool to no avail. General error is:

System.ArgumentException: Error creating a repository for: EntityTypeName: Foo, Exception has been thrown by the target of an invocation., System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: Must specify valid information for parsing in the string.

The endpoint is:

https://www.client.org/Asi.Scheduler_clientimis/api/Foo?Limit=1

Any idea? Thanks.