We are in the process of adding Lightning Support to a large Managed Package and have run into an internal Salesforce Error for one type of page. 

As background, part of our package produces complex documents (e.g. Invoices, etc.).   These are produced via Visual Force Pages, and are presented in an iFrame.   The outer frame supports commands for printing/saving/etc.   The inner iFrame contains the Visualforce that creates and presents the complex logic.

In lightning we are getting an internal Salesforce error from the internal iFrame.   The error is:

There was a problem saving the PDF file. The error was:

System.VisualforceException: Failed on parsing CSS sheet at https://na52.salesforce.com/slds/css/1SusXNPYqeWuyHSnfU7K8g/min/lightningstylesheets/one:oneNamespace,force:sldsTokens,force:base,force:oneSalesforceSkin,force:levelTwoDensity,force:themeTokens,force:formFactorLarge/slds.css

Naturally, we will not be able to adopt Lightning until this is resolved.   Any thoughts?