Thank you all for your support and help.
I haven't been able to find out the cause of the problem yet, but I don't think it's related to FPC/Lazarus.
I installed the r62663 on another PC, always with Windows 10 64bit [WIP] but with a new AMD CPU, everything works here properly; the PC where I had tested before has an Intel CPU. Of course this is not the definitive proof: As we all know it is difficult to find two PCs with the same configurations; so I will have to do further tests. If I find out the causes, I will inform you.
Greetings.
Otto.