The target-entity "some entity" cannot be found

In our case, the file name was not the same as the class name: it was just a typo.

Your entity declaration is incorrect:

 * @ORM\ManyToOne(targetEntity="Entity\User", inversedBy="subjects")

This should be either this:

 * @ORM\ManyToOne(targetEntity="Subject\Entity\User", inversedBy="subjects")

Or, since the two classes share the same namespace, you can also use this:

 * @ORM\ManyToOne(targetEntity="User", inversedBy="subjects")

The targetEntity has to be the fully qualified class name (FQCN), except if referring to a class in the same namespace, in which case the short name may be used (as per the last example above).