App only works after restart

I built my first flexdashboard. It works great locally and I can deploy it to

There the flexdashboard appears in the browser but all plots, maps, and tables do not show up. Everything else (value boxes, input selectors) is displayed correctly and there are no error messages (also not in the logs). The weird thing is that sometimes the flexdashboard loads completely with everything displayed and I found out that this works once every time I restart the App from the dashboard - but only once. If I want to open the App again, I have to restart it.

I tried to find reasons for this behavior of the app but could not find any helpful info.
Thank you in advance for your suggestions!