I also experienced this problem, and I had to operate the panel without a domain. Back in the day, it was super-easy to do without running into problems. All you need to do is enable sub domain SSL before assigning panel sub domain under settings. Aapanel would give you the option to include SSL, and that was all.
Last month I tried it, and it failed because aaPanel now assigns a local SSL by default when installing the panel. Once you assign Let's Encrypt, it doesn't replace the local self-assigned SSL with Let's Encrypt.
I copied-paste the Let's Encrypt SSL from the sub-domain under list of websites and pasted it under Panel SSL section. Do this before assigning the Panel domain. Once you do this, restart the aaPanel via SSH.