Inaccessible oc-upgrader.phpon while trying to update from 2.3.3 to 2.3.4

[Bubstar]

Hi there!
I'm trying to update my 2.3.3 board the 2.3.4, but no metter wha am I doing I'm getting always the same error

You don't have permission to access /install/oc-upgrader.phpon this server.
Click to expand...

I have full(Root) access to the server, tried even to Chmod the inaccessible file to 775, but that's no metter too.
I've also tried to do the update on the local test version of my board that I have, but there I'm getting the same error and I really don't got it.
Any ideas please?
Inaccessible oc-upgrader.phpon while trying to update from 2.3.3 to 2.3.4

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