Finally I found the "root of evil". My previous thought it was based in a random event as it seems.
I'm pretty sure the problem is lying at the function public.get_path_size()
On this specific VM I use a mounted Hetzner storage for backups. The VM is located in USA while Hetzner storage in Germany and the connection is slow, especially when the network of the hosting company is overused.
If replace the function on the call from get_panel_backup_info() at publicModel.py like "result["total_size"] += 1" instead of "result["total_size"] += public.get_path_size(f_path)" it works fine... with a small delay (because of os.path.isdir(f_path) but at least it works.
Of course, I don't know what impact this change will have on other parts of Aapanel, but at least now I have a responsive interface and I can manage the server.
I'll really appreciate any solution other than "change hosting company" 😉