Create more verbose errors when exporting to PDF

[RTPL_AU]

Using Pro 3.5.1

Something in a layout or map upset Pro and when exporting the layout to pdf I get this:

and the warnings are:

Please make warnings more verbose so that we have some idea as to where to start looking for things that could possibly be unsuitable according to Pro.

As far as I know every symbol/style/font/dataset in the map & layout has been used before. 
I don't have the time to go hunting under every rock to hope to find a glass spider.