Hi, Tijs Rademakers.
The way is clear (it reprodeses always). I created new Activiti project and added two simple linear diagrams (D1 and D2).
Then I added CallActivity on D1 and choosed process from D2 (via popup dialog by the field "Called element").
The choosen process' name was displayed in the field "Called element".
Then I switched into "General" and returned back to "Main config".
The field "Called element" was empty.
Yes, I'm on Windows 7 64-bit.
Best regards.
Sergey