In a check for safe character (Jenkins.java) there's a big list of disallowed symbols.
I don't know what's behind that restriction, probably you cannot use : as a job names because on windows the character is illegal to have in path component.

Currently i want to make a node name like a@b, and the system says the character is unsafe. Why?

Likewise ＠ (fullwidth commercial at — U+FF20) doesn't work. I can create the node name with it, but cannot tie a project to this node because at this step it claims the character is illegal.