dim s as new shell
In other words, Xojo indeed points to the proper folder. So it is indeed Catalina not behaving like previous macOS which allowed access to the temporary folder.
For some reason, Catalina prevents Xojo apps to access the temporary folder.
I opened the temporary folder from the command line with
And dropped a file in there. No error. So it would seem the user can access that folder, but not a program.
The plot thickens...