Ok, I used the 9V battery from my other multimeter and in fact the problem is over. Anyway, this shouldn't work this way in my opinion.
Anyway, problem solved.
However I have another question. Not sure if I should start a new thread or just change the title of this one. Anyway, I'll ask here.
Anyone knows the purpose of that kind of mount (or whatever that can be called) on top of the back side of the multimeter?
Well thats good.
I have 1 too, and AC and Freq readings stopped working, and I thought I damaged it by overloading the mVDC or mVAC mode. But it just drains the battery. I took it completely apart, and started mapping out the circuit (I'll finish it some day) . But it was just the low battery. And when I powered it off the PSU, I somehow missed it was working, because I had a hard time with the dial, when it was apart, and sometimes w/o the display, upper PCB/etc.
There's also a trick to starting it when it's in pieces, related to the sleep mode, and where the dail was or is or something.