HRM Run connection problem

I suddenly have a problem connecting my HRM Run to my 5s+. It worked fine until about two weeks ago, then the HRM was suddenly not recognised anymore. I replaced the battery but it still wasn't recognised so I assumed the HRM was broken and order a new one. Annoyingly the new HRM Run also wasn't recognised, at which point I decided to try to connect the heart rate monitors to my old forerunner watch. And they were both connecting.

Does anybody have an idea what might be going on? I'm on the latest official firmware v6.0 and sensor hub 2.30.