I am having the following problem:

I have a block with preinstructions, trials and a timeout. The timeout is set to 90 seconds. According to the documentation, this time should not include the time spent on the instruction pages. But measuring with a watch shows that the timeout occurs after 90 seconds including the time spent on the preinstructions.

Is this a bug in Inquisit or am I doing something wrong?

This is correct, AFAIK. In other words: This sounds like there may be a bug involved. As a workaround, you should be able to put the instruction <page>s in separate <block>s and then run stuff sequentially:

<expt>
/ blocks = [1=instrblock; 2=myblock]
</expt>

<block instrblock>
/ preinstructions = (mypage)
</block>

<block myblock>
/ timeout = 10000
/ trials = [1=mytrial]
</block>

<page mypage>
bla
</page>

<trial mytrial>
/ validresponse = (57)
</trial>

Thanks, it works now! :)