Why the old site is not working (or just me not working)

I'm not seeing any issues with it. What errors are you having?

I get an error in a red rectangle when I try to create a focus tree

Yeah, the old tool still works. But the warning is saying that any issues with the tool are not supported on hoi4modding.com. Only the focus tree tool on this site is supported, as it is the most up to date version.