OK, after doing a lot of tests (thanks @chris for ruling things out, and getting me on the right path!), I now know what is the issue in my system. Between M4 and RC1, there must have been a change how OH treats binding startup failures: I had “http” in the bindings line in my services/addons.cfg that did not work. I guess it should be “http1” nowadays(?). Anyway, in M4, this just gave an error in the log file, but in RC1 (and M5 I guess), this makes OH reset/restart all(?)/most bindings once every minute, including Z-wave binding, which in my case is very large/easily spotted.
After removing “http” from my binding line, I have run without any issues. When I put it back, back comes the issues.
This might be useful to know, and maybe also this explains other peoples issues.