Thanks @Aleks. I managed to restore mysql, drop databases and reset default mysql password.
I tried different commands to reset my Yunohost instance. After a lot of attempts I finally managed to restore the Yunohost backup. It works again.
After this restore action I still have a problem with dyndns., the answer might be in this thread. I’ll check this one tonight.
Cron <root@xxx> yunohost dyndns update >> /dev/null
No domain has been registered with DynDNS