What type of hardware are you using: Old laptop or computer
What YunoHost version are you running: 11.3.0.2
How are you able to access your server: SSH
Are you in a special context or did you perform specific tweaking on your YunoHost instance ?: no tweaking
When upgrading to yunohost 12, I get an issue/error message about fetchmail daemon not starting 
logfiles here: https://paste.yunohost.org/raw/ufibucepos
I don’t understand why fetchmail need be installed on your system ? What is the list of apps in your instance ?
yunohost app list
Same here, dunno why fetchmail would be installed, what does it yield to simulate an uninstallation using sudo apt remove fetchmail --dry-run ?
Indeed, I installed fetchmail to collect mails for other mail accounts.
Removed and re-installed it and this issue was gone.
upgrade still has some issues, but this will go to another thread.