Press the correct 5 key.
There are two 5 keys on your keyboard, one at position E05 and one on the numeric keypad, wherever that happens to be (either a separate keypad or an overlain keypad accessible with a Fn key or similar). You are pressing the wrong one.
It does not matter that you have a Hungarian keyboard. The engravings on the keytops do not control what actions the keys cause. What matters is that you are using a Hungarian keyboard mapping such as
hu101.kmap (on Linux operating systems).
In that keyboard mapping only one of the 5 keys produces the
␝ character in conjunction with the ⎈ Control modifier, the key at position E05.
␝ is of course the local escape sequence for the
In that keyboard mapping the other 5 key, on the numeric keypad, produces the XTerm control sequence CSI
u that you are seeing; which denotes the 5 key on the numeric keypad, in "application mode", occurring once, with the ⎈ Control modifier.
As the Hungarian keyboard map says:
control keycode 6 = Control_bracketright
keycode 76 = KP_5