In a second step, I wanted to include scores of a questionnaire that were collected from participants before the experiment and are believed to influence the dependent variable as well.
There is one questionnaire score for each subject (indicating how stressed they were before the experiment)
is this a fixed or a random effect?
The questionnaire score should be a fixed effect in your model, unless it is a mediator, in which case (assuming you are interested in the total effect of condition, and not just the direct effect) it should not be in the model at all. It is always a good idea to draw a DAG (ie., a causal diagram) which informs the set of variables to include in a model. See my answer in the following thread for details: