Thank you @Integer Mike for your quick reply. In my case refresh does not help.
Your reply motivated me to deeper analyze the behavior.
When I open the overview page (default start page without “/page/…” in the URL) the icons are displayed even after I do a refresh (I first thought that they show up because they are stored in the cache).
If I open the same overview page or any other page from the sidebar (with “/page/…” in the URL) the icons are not displayed
Within popup pages the icons are not displayed
when I do a refresh on a popup page I get an error message
The problem seems to be related to the OH build-in webserver (URL, timeout, …?)
F7 icons work fine, only icons stored on the OH build-in webserver do not show up (icons stored in the folder /etc/openhab/html/icons … SMB share: \\openhab\openHAB-conf\html\icons ).