Just had this reply from Silverpop on the issue (an impressive 2 hour response time!):

28/01/2016

Hi Peter,

Thank you for contacting Support at Silverpop, an IBM Company. My name is Michael and I will be assisting you with this case.

The format of Refresh Token has been changed from 36 characters to 46 alphanumeric characters in August 2015.

As you have created the new refresh token, you will need to contact EpiServer regarding the documentation and actions required from EpiServer.

Kind regards,

Michael D'Souza
Client Support Specialist | Silverpop, an IBM Company

Over to you guys at EPiServer.....  

Good catch. I've notified the relevant people over at Episerver. 

Peter, according to the Add-Ons team this was fixed months ago (probably around the same time Silverpop changed it). If you upgrade to the latest version of the connector you should get it running again.

Arild, thanks for your help.    :)

(I'm assuming it's normal for you to reply yourself, rather than the add-ons team?)

Anyway... I'm a little wary of making changes to Silverpop setup as it was configured at least a year ago, by a competent developer who has now left, and I know he found it 'fiddly'. I'm not sure how it was installed (NuGet maybe?), and when I look at the solution... there is a strange looking zipped file in folder: '\modules\_protected\EPiServer.ConnectForMarketingAutomation' - that is called 'EPiServer.ConnectForMarketingAutomation.zip' ... seems weird to me it is zipped? Is this it?

Sorry if this sounds childish, but could you or the team give me a brief outline of the process to upgrade & where do I go to to get newer version?

cheers

Episerver are more than capable of answering for themselves, but since these are user-driven forums, Episerver developers aren't constantly monitoring them. Still, bugs relating to official add-ons should be elevated to the right people asap to avoid downtime for customers.

The file you're referring to would be the add-on, yes. Just leave it be, it will be replaced when upgrading.

It should be easy to upgrade the add-on using nuget (requires the help of a developer). Just open your Episerver project in Visual Studio, and install the add-on nuget package through the Nuget Package Manager. 

Once installed, you might need to review your configuration.

Refer to the documentation for configuring Silverpop, or using Silverpop. 

Beware that the very latest version of the connector requires Episerver 9, but there are nuget versions available for Episerver 8 also (just check the dropdown in the NUget Package Manager to view the dependencies).

Unfortunately the only relevant connect packages listed have a dependency on Framework 9. We are using 7.5 ( ver. 7.19.2.0).  

We naturally don't want to upgrade the whole site just to accommodate the latest greatest versionof this add-on.... would introduce too many risks at hthis late stage.   :(

If a change ocurs that makes a software add-on break (which the new GUID format does)... then an incremental upgrade of that tool should be made available, that works with exactly the same dependencies. If the only possibility is to upgrade entire site from 7.5 to 9 ... that seems pretty poor service to me.

I have raised a ticket with EPiServer to ask if they have specific .dlls available that might fix this...

https://episerver.zendesk.com/hc/en-us/requests/32272