Today I found that one account having weird error.
1) The group ownership became an ID and the site is unable to load as in valid group.
2) Where try chown the folder, it will return “invalid user” error
1) check if the group ownership is correct, it wrong as below, please continue step (2)
2) open /etc/group and add in USER:x:GROUP:
For the screen shot case, add it as ipless:x:512:
3) /etc/gshadow and add in USER:!::
For the screen shot case, add it as ipless:!::
Done and the group permission will correct automatically.