One of my informants says that the following sentence is not quite right because it is not an action that her boss could cause. That is, it is not good to add "by her boss".
She was obliged to resign by her boss.
Why are there cases where by phrases are not allowed?
You can be obliged to do something but you cannot be obliged by someone to do something. The obligation does come from someone or something outside of yourself but being obliged refers only to the feeling in yourself.
A better way would be to use required by ie Students were required by the professor...
It seems like a prescriptive chestnut to me. I think you can get away with it with no great complaint nowadays.
There is still no consensus over the issue of a Syrian military withdrawal from Lebanon — which the Christian MPs have been obliged by their militia leaders and by General Michel Aoun to discuss, although the Muslims are reluctant to debate the topic at pre
However, 'She makes me laugh' is much, much more common.
(Incidentally, I still cannot find any reputable reference to the rule that 'obliged by' cannot be followed by a person. Strunk & White do not mention it, nor do Modern English Usage (Fowler), the M-W Dictionary of English Usage, or on-line dictionaries.)