A couple of weeks ago, I noticed that hyperlinks in a Notes mail do no longer open. I did not pay much attention to it, because I am reading my mails mostly in the browser or on a smart device.
Today, I did some Google research. I came across an old technote that best describes the behaviour.
LO56433: CANNOT CLICK URLS IN NOTES 8.5.2 WITH OS BROWSER
What I found is that HKEY_CLASSES_ROOT\https\shell\open\command\ pointed to a non existing browser and HKCU\Software\Classes\http\shell\open\command\ was completely missing in the system registry.
Setting the entries to the DEFAULT iexplore.exe has fixed the issue.