Welcome, @maschwab !!
I was able to reproduce this yesterday, and it eventually started. It seems the cluster that runs Rstudio is very busy, and the environments are ending up queued for a while. Also, remember that you can run just one at a time at the public sites.
For this part
Unfortunately, in the past this has meant that the session is lost… BUT let’s ask the EU administrators of they have any (better?) feedback for you. ping @wm75 or maybe @bjoern.gruening