Parece que você esqueceu de usar a opção -a
para usermod, então ela apenas alterou o grupo suplementar de www-data para o ubuntu e, em seguida, removeu www-data de todos os outros grupos suplementares. Você precisará adicionar a conta www-data novamente nos grupos suplementares relevantes
usermod -a -G <group1>, <group2> ...
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]] A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option.
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list