Display hyperlink error on Close button

The DisplayHyperlink in my workflow opens up a report in PDF format. The report usually opens up OK (although sometimes it produces a completely blank page which appears to be random, and I haven't yet worked out why). The main issue I have is when clicking on the Close button of the hyperlink container, I get an error:

 

 

'There was a workflow error running activity: Exception has been thrown by the target of an invocation. Unable to load specified workflow from instance store, Unable to load workflow running instance data. There is an error in the XML document. Parameter is not valid.'

 

 

However, I do not get an error if clicking on the 'X' to close the container.

 

I found a post from Dec 2018 which had a similar issue, but I already have a separate data table to store an exported bitmap which is the basis for the map in my report.

 

 

https://support.geocortex.com/essentialsGSCForumDetail?&id=0D5f200006aOfEpCAK&dc=&sub-nav=forum&main-nav=essentials

 

 

I had no problems in using the close button in earlier stages of testing where I didn't try to include a map image in the report.

 

 

I am using the classic Workflow Designer v4.12.0.64