Running the Content Library Explorer tool, I received the error ‘The legacy package does not have exactly one content’.
I had seen this before and the fix was to run a PowerShell script to report back problem packages – see https://blog.basevision.ch/2014/12/error-connecting-to-distribution-point-with-content-library-explorer/ and fix the packages accordingly.
On this occasion nothing was reported back as duplicate and all folders were unique.
The fix was to run the Content Library Cleanup Tool (initially in /whatif mode BTW!) and fix/delete the problem packages.
As soon as the clean up tool had completed, I was able to load up the Content Library Explorer tool on my effected distribution point.