is it safe to run do-release-upgrade from ubuntu 18.04 to upgrade to 20.04 with ispcofig running fine on the existing server?
It is a normal process and should be safe provided you follow both ISPConfig and the OS update steps properly after doing a good backup for that server. Another safe option is to use ISPConfig migration tool. Alternatively, if you subscribed to Ubuntu ESM, I think you may also opt not to upgrade it until before 2028, that is if ISPConfig still support for that version until then. I always choose the former notwithstanding the availability and subscription of Ubuntu ESM because I am not sure ISPConfig will keep it support for it until before 2028.
