OH2 on RaspberryPi 3 is not updating GUI after changes to sitemap

Hi all,
I am a very (very) beginner with openHAB2 (and openHAB in general), but I managed to install it on my RaspberryPi 3 quite well (using apt-get) and now I am in the process of learning how to configure it with items, sitemaps and so on.
As a start I try to get the Raspberry Pi system temperature example up and running (https://github.com/openhab/openhab/wiki/Raspberry-Pi-System-Temperature). So far with not too much luck.
After fixing some typos I get CPU and GPU temperature displayed, but still no chart.
But that is not my biggest issue. What really anoys me is that I have to restart openHAB2 everytime I make a change to the sitemap. Otherwise the change is not visible. Clicking “reload” page or shift+reload does not help.

I use chromium on a 64 bit Arch Linux installation connected over WLAN an a router to my Raspi.

The logfiles show no errors, only [INFO] entries in /var/log/openhab2/openhab.log

Is there any special command I can use on the karaf console to force openHAB2 to invalidate some caches?

Do I miss some debug / log outputs that could give me hints where to look at in my configuration files?

I am happy with any pointer or help where to look at. Thank you all a lot in advance.