Just FYI, I had the same decimal.ConversionSyntax errors and was able to resolve them by downgrading my linuxserver/mariadb to 10.11.10 from latest (11.4.4). Prior to downgrade I dumped everything with mariadb-dump and was able to restore using the dump sql file pretty easily. I was also prompted to run mariadb-upgrade on the 10.11.10 version, which I think I did after restoring.