I had the same issue on a M1-Max Studio (with installations of 26.4 beta 3, beta 4, and the RC). I finally solved it and reported the following as a appendix to FB22153797: SUCCESS!! First, what didn’t work — I booted in Safe Mode and tried to install Tahoe 26.4. This failed just as reported above. Then, I DEINSTALLED the F-Secure security suite (which came with my Spectrum Business ISP account). Then attempted the installation (in NORMAL mode, not Safe Mode). The installation worked. So it is clear that the install problem was related to something that F-Secure did to my system that was undone by de-installation.

Thank you. That's what I was hoping you'd say. Cheers

Thank you. I get it. I implemented a call to .fileExporter in the MacCatalyst case and it is an acceptable solution — only one extra mouse click. However, I would like to point out that I'm not "littering the Documents folder with random things by default". In this case, the user chooses a "Save to File" option, which causes a carefully curated folder of information to be zipped and placed in their Documents folder for the purpose of being accessible for sharing with others - for example, so they can place it in Google Docs or Dropbox. (I also implement a "Send as an email attachment" option...) ...just sayin'