strange 3 hours ago it worked without any problem. i locked and unlocked it 4 times and it worked just like it should and now its not working again.

Yes, as mentioned in the BackupManu thread, BM uses that switch as the reset button.

That just confirms the transient nature of your problem. Moisture of dirt.

i think i should take it to repair shop. they told me today bring it tomorrow and i will repair. this will cost me about 3 to 5$.

its now working again. seems like it was moisture as Pichlo said. thanks everyone for your replies. and sorry for posting so late and changing thread title to solve.

I'm happy I've come across this thread.

I'm having the exact same issue as panjgoori. The screen started to flicker today (even in my pocket, got a lot of apps opened and shortcuts deleted by itself). It will often flicker by it's own : I mean the phone is on the table and the screen unlocks by itself and then 1 second later locks and again unlocks (like a lot of times) and then just locks. And also the battery is draining really really fast because of the flickering.
And the same thing as described by panjgoori happens with backupmenu, it restarts after the menu is showed.
I didn't drop it lately, I mean the last time was like 6 months ago and it worked fine until today.
I hope it is moisture, but in the country I am no repair shop wants to open it to clean it. They say it is too complicated because of the slider mechanism and it is not worth it. I would do it myself but I don't have the necessary skills.
This weekend I will try some other repair shops and I hope someone will be willing to take a look at it.

You do need some basic skills but opening is easy-peasy for anyone who has opened as much as a transistor radio. It really should be no challenge for a repair shop.

They know they can't ask a premium price for a dated phone like the N900 so don't even bother.

https://www.youtube.com/watch?v=aVEHm325-z8

So after going at like 10-12 repair shops (don't remember actually as I lost all my weekend on it) nobody wants to open and clean my N900.
They all give the same response : it is an old phone and it doesn't worth the effort from them to deal with it cause they will have to do some research and stuff.... they are just a bunch of lazy.

So, what I will do : first buy a second N900 (as this one was my main and only phone, got to replace it with an old nokia phone for now), then get the necessary tools (will have to look for what I need exactly) and open it myself. I never had to deal with opening a phone (not even a laptop) until now, but like they say "you have to start some day".

Thanks t-b for the link it will certainly help me.
Also, I will have to recover my backup from backupmenu on the n900 I will buy next week (if I find one worth buying), but I've seen on the forum that it is discouraged to recover a backup on another device (but I don't remember why).
I hope it is doable as that was the intend of the backups I did, to use them on another n900 when the one I have will fail.

So, I just got an used N900 (but in very good condition) for about 55 euros. Now I have to reinstall every app (including catalogs and all) to customize it the way I want (it may take a while).
I ditched the idea of using the backup from backupmenu as I would like to have a fresh install with all the configurations.
I will try to open the old N900 and clean it just to have a backup (I hope I won't break anything and that I will be able to assemble it again).

Restoring a backupmenu backup from device A on device B is a lottery. I've heard success but also horror stories. Here is mine.

Good luck with your hacking, I am sure you will come out richer for the experience.