The pre-release clean Install was SUCCESSFUL
What I did
1. Installed the latest macOS Big Sur 11.3 update - I don't believe this has something to do with the issue.
2. Uninstalled the CommandLineTools by deleting the directory /Library/Developer/CommandLineTools
3. Installed again the CommandLineTools by executing command xcode-select --install
4. Run clean install for fpc trunk.git + laz trunk.git using the FpcUpDeluxe pre-release
Result: Successful build
Most probable the reinstalling the CommandLineTools solved the issue. It is possible also the pre-release changes to be part of solution too.
Thank you, @DonAlfredo, @trev, @Rainbow6 for the help.
@DonAlfredo, if you need me in the future, for pre-release testing, for design or code reviews or anything else, I'll be glad to help you.