In a 12/8 meter, I am trying to write two eight rests as a fourth rest in a percussion part. Even with the force duration tool it doesn’t seem to work. How can this be solved?
You need not only Force Duration, but also Rest mode (,). In Rest mode, you can entered forced durations of rests.
Bert, 
for the first I used the comma only, for the second, force duration as well. Frankly nothing strange or peculiar…
I think it’s also worth a look to Engraving and Notation Options…
The problem might be caused by my computer keyboard. It has the bad habit to changing into qwerty and even when I reset this, it keeps acting strange. Is there a way to access rest mode without the komma? If I press the komma sign now, it doesn’t respond.
Is there no setting in which I can make all two eighth rests in 12/8 turn automatically in a fourth rest? If I would write it myself, I would never write two eighth rests in this context.
Are you in Note entry mode (shift-n, the caret should show)?
You can’t currently interact with rests on percussion kit staves. This is something that’s being worked on for the next major version of Dorico.
In the meantime, if you really need to fake a quarter rest then you can do by building a notehead set that uses rest glyphs in place of noteheads - see original beaming, how? - #3 by Rob_Tuley for instance.