Aternos servers are all like this, and this isn't the place to ask for support. This is a Minecraft bugtracker, and as such should only be used for bug reports.
@slicedlime I found the log. It said something about 'launcher already open,' and apparently rebooting fixed it.
I haven't the faintest idea why.
Thanks just the same!
(By the way... does the new launcher use Electron, perchance?)
This was exactly my experience on a 32-bit computer. It's either a library incompatibility or just that Minecraft doesn't support the 32-bit architecture anymore past version 1.12.2.
Either upgrade to a 64-bit OS / computer, or re-install Minecraft in 64-bit mode if that's possible.