Hey Sebastian!
As a commercial customer supporting all of our open source work, always feel free to reach out to support@rstudio.com with questions such as this one. You also have an account representative that would be happy to route your request, too.
In any case, you can access the docs for different versions at docs.rstudio.com, although it looks like 1.5.2 is old enough that we did not follow this pattern: http://docs.rstudio.com/shiny-server/1.5.3/
Also, we do not technically support 1.5.2 anymore (now that we are on 1.5.7), so we would definitely encourage you to upgrade as soon as you are able!
I'm not super familiar with the ins and outs of the utilization scheduler load factor, to be honest, but I would try something nonzero. Maybe something like:
utilization_scheduler 1 0.9 10;
See if that will spawn some new processes for you! You will need two or more different browser sessions to connect to the application simultaneously.