The message is “Exception in Run: The handle is invalid”
Can you please enable audit level logging on the Player. That should give a second log entry there which will give us a stack trace.
Please can you also confirm that you get a new log entry every time you request a screenshot?
Yes, new log entry every time screenshot is requested.
May need clarification on enabling audit level logging on player. I tried from advanced tab on edit display (is that the right place? - see below) - but did not receive a second log entry when requesting screenshot.
Auditing there is CMS side auditing for the display.
To put the Player itself in to audit mode, you need to create a Display Settings Profile for it, with Log Level (on the Troubleshooting tab) set to Audit.
You then need to assign that profile to that display, and wait for the settings to be updated (one or two collection intervals).
I’m afraid that didn’t quite do what I was hoping it would - which was print a stack trace (it just printed the exception name). I’ve logged a comment on the bug report and will provide a version that does that when I can.