Notices

/home/user # cat ~/.config/user-dirs.dirs
cat: can't open '/home/user/.config/user-dirs.dirs': No such file or directory

Then you may create such file with following content (output from ~ $ cat .config/user-dirs.dirs):
--edit
Just seen, that you executed as root.
Under root's config there is no such file. Please try again to check as user.

I get the same message, when I am not root.

Edit: Damn, just noticed the full path in your post which makes most of the below irrelevant but I've decided to leave it in case some noob follows your footsteps.

~ stands for home directory and is different if you are user or root.
For user, ~ is /home/user. To avoid confusion, try
The easiest way to get there is by using automatic expansion. Type cat, space, slash, h, then click Tab in the X Terminal tool bar (to expand h to home/), u, Tab again, dot, co, Tab, u, Tab. If that does not expand to /home/user/.config/user-dirs.dirs then you are in trouble. You can copy the text in the code block in peterleinchen's post to a file, name the file user-dirs.dirs and copy it to /home/user/.config, but I wonder if that file is missing what else is.

Then proceed and create that file (possibly a reboot is necessary, idk).

--edit
see also pichlos advice...

Many thanks to peterleinchen and pichlo!

I am a little bit confused, beause your advices did not work. Therefore I openend "Filebox" and made a screenshot from "/home/user/.config":



On the picture you can see five files. These are the only files within the folder ".config". Am I right, that there is no file called "user-dirs.dirs". Do I have to create this file now? For example in "Leafpad" with this code from pererleinchen:

Yes!

10 chars or

Great! It is working now. I am something like a stupid user! But I managed it with your help.

Something has changed now! Till I did this, in the N900's file manager I could see on "MyDocs" a folder called "DCIM", but every picture was saved at "MyDocs". After doing this, I do not see the "DCIM" folder in the N900's file manager, but the pictures are beeing saved in the folder "Kamera", now. I think (and hope) this is right.

Thanks again!!

That is expected. The config file user-dirs.dirs masks the true directory names so that the stock file manager will call them something else. What exactly depends on your theme and language. For example for me, the DCIM folder is called Camera. .sounds is called Audio Clips etc.

I am a bit concerned that your ~/.config/ contains only five items. Mine contains 34 including hildon-desktop, osso-calculator and tracker. I wonder how customized your device must be to work without those.

Do not make me nervous

On my picture, you can see only "files" directly laying within my ".config" folder. But there are more than 30 subfolders, as well. I hope, this is ok.

Otherwise: If a stupid user like me is playing with his N900 (following all the nice threads here ), mystery things can happen.