Failure in retrieving a bibtex file from a zotero group

I'm trying to create a bibtex file from a zotero group but keep getting the following error message

BibBase: Error retrieving bibtex file.
Request was:

https://api.zotero.org/groups/2059934/items?key=9mrXH5RoOmSNchCUQOYGSVKJ&format=bibtex&limit=100
{ 'User-Agent': 'bibbase' }

I have no problem in retrieving files from other zotero collections.