Maybe I finally managed to solve this.
After yet another failed attempt with the last release I made a desperate attempt and started my Debian Linux with “nosmt” option in the bootloader, disabling hyper-threading.
This way, the build process shows only the actual 12 physical cores and not the logical ones. The build with all 12 cores in parallel went well almost to the end, then the build process was stuck with 1 core at 100% forcing me to restart the PC.
After the restart I relaunched make cmd and the compilation was successful without any other hitches.
U5 finally starts without problems!
It took me about a year to figure out that it was some kind of issue about clang/threading/cores…
Finally I can go back to work decently, I hope that this can be of some help to others too.