The connection was reset


Using the TO for a while now with no serious issues.
One thing bugs me. After some 2-3 weeks after a reset (on-off-on) i can’t reach Luci or Foris anymore. Al other websites work just fine.
This happens with all of my browsers (FF, MS Edge and IE under W10).
FF displays " The connection was reset" and MS Edge/IE “can’t reach this page” (errorcode INET_E_DOWNLOAD_FAILURE).
I tried different types of reloads, clearing the cache, deleting cookies. Nothing seems to help, only a reset of TO. Does anybody know what is going on?