Saving edits in recurring events

Hi
The last few days I've noticed an issue with saving edits made to recurring events. i make changes to the parent, but the green progress bar never moves...and the changes aren't made.

If I refresh the 'saving page' I then get this a joomla error
"An error has occurred
-1 Source Folder Not Found
Return to control panel"

If it does update, then the updates across all the events are confined to text alterations. Changing a cover image or icon doesn't update.

The IT courses in the Education/Courses category has some examples of recurring events this month where I've discovered this issue.

I haven't had a problem this before...it's only since the server php7 upgrade.

Thanks
Paul