No icons visible

I’m using Piwik since eight month approx. on my own Debian 8 server with Apache 2.4.10, PHP 5.6.24 and 10.0.26-MariaDB.
Since the last month I can’t use the webinterface anymore in the right way because all icons and some other symbols are missing. See the screenshot below:

Do you have an idea why?

Unfortunately there are no replies. Are there any glues?

I would do a view source on the page to see where it is looking for those images, then I would check to see the images are in the correct place and the permissions are set correctly. I would also check the Apache and PHP logs for any errors.