As Mauro said: "you have to remove and re-add the Eclipse Project Update site, so that its metadata are re-calculated." - works as workaround
Another workaround I found, is to edit the pre-defined URL link by adding just a trailing “/” to the update site URL.
The third workaround I discoverd accidentaly is to do nothing, but to uncheck the 'Contact all update sites during install to find required software' before checking the URL link.
Option #2 worked for me. I went to Window > Preferences > Install/Update > Available Software Sites, then for each enabled site I added a / to the end of the URL (if it wasn't there already), then clicked Reload.