Last diagnosis was it looks to be either the circuits being pulled down or still suffering from a regulation issue. The 6V out of a 5V regulator is a concern because between ground and out should read 5V.
By measuring between the rails and ground you can determine if a low resistance path exists which could be loading the output. The other idea is to isolate the transformer and feedback circuit and inject DC or AC to check the regulation, in a similar way that you did it out of circuit. Or check component by component. When the regulators or diodes are out it allows you to easily measure the input side of the regulator to ground or the output side to ground.
To put it clearer, not a lot of components in that circuit should be drawing sizable current to ground, only the regulator which has to do it in order to lower the voltage. Aside from the diodes it's just the resistors in the feedback circuit.
Check the Y suppression caps (marked CY01 CY02) as well.
Confirm DC regulation is perfect, confirm secondary components not applying adverse load or affecting feedback, confirm optocoupler is functioning correctly and affecting switcher. Confirm switcher and flyback is performing within spec.