@Michel B I am going to investigate further, but one of my apps is apparently unable to write to the temporary folder under Catalina.
There do appear to be some weird problems with the temporary folder in Catalina when older file system APIs are being used. Basic testing seems to indicate that Xojo 2019r2 would address both issues.
These are the two issues we are seeing:
1. Files written to the temporary folder by application 'A' cannot be read by application 'B'.
The console reports a sandbox violation error which according to Apple should not be happening.
2. Files copied into the temporary folder that have extended attributes such as com.apple.FinderInfo cannot be accessed correctly afterwards.
Up until beta 9 we were seeing sandbox violation errors if the application / file with the attributes wasn't in the Applications folder. With beta 9 / release, the problem is worse as it does not matter where the file is located and the sandbox violation has been replaced by some kind of read failure.
Both problems were reported to Apple during the beta programme with the second not even getting a single reply. I guess they don't really care about compatibility / stability any more.