How to get more space by manually deleting unnecessary system files (rooted S2)? I keep getting warning that apps cannot be installed on their default location since there is too little space. 

However, when I check in system there is a bunch of space, especially for small apps of a few MBs. 

 
Screenshots (click for larger variants)

I tried moving some apps on SD card, but again I got a message that there is not space available. 

Is there some system space occupied by unnecessary files which I can manually delete?

Or other way to solve this silly bug?

This is Galaxy S2 with official 4.1 OS. I have root access (rooted phone).
Aside from Izzy's suggestions, I also find Clean Master helpful in cleaning unecessary files / cache. Check it out. :)