Hey!
I had the same problem and after some playing around with the code I finally saw that the SOAP Endpoint URL that was filled in the settings page prior the appearance of the blank page had a leading whitespace. The code doesn’t bother to take care of such things and feeds it to the SOAP object, which results in an error and break the entire execution. It’s not even enclosed in a try/catch block.
So, you can either make a temporary fix in the code in order to display the settings page and remove the unnecessary blank space or you can make the change in the database. The settings are stored in wp_options with option_name “gf_marketo_settings”. Keep in mind that it’s a serialized array, so you need to account for the string sizes. Alternatively, you can just delete the entire entry and re-fill the data, watching out for the proper typing of the URL.
I hope this solves your problems, as well.
