I managed to add the folder to Paperpile by setting up a new Mendeley account online, uploading the folder to Mendeley (containing 263 items), and then importing into Paperpile. In Paperpile, there were only 262 items. This is not because one item was duplicated, because many more than one items are duplicates.
After exporting citations to csv and comparing with refworks exports, I wonder if this problem was caused by a paper titled:
** Economic and Social Impact of Increasing Uptake of Cardiac Rehabilitation Servicesâ€“A Cost Benefit Analysis**
This is a duplicate paper. I have another one in the same file (of 263 citations) with a title containing only standard letters.
I can't tell for sure unfortunately, but that is my best guess as to what the missing paper is - and therefore what the problem with uploading the RIS / BIB file was. This title with unusual characters is the only difference between my refworks library and the Paperpile folder when I export both and compare.
The interesting thing is that I can't find the paper with unusual characters (pasted above) in my refworks library online, although it is there when I export to excel. Although reworks online says that it has 263 references, my refworks library only shows the title without special characters when I search for keywords/authors.
I feel satisfied that I haven't lost a useful paper so am able to move on with my review. I have no idea if any of this is relevant to share but maybe it counts as a bug I experienced now rather than a support request.