Ok, I think I managed to resolve this issue. My guess is that it is due to the fact that the myopenhab bundle is loaded last and thus the rule parser does not know about its sendNotification action.
I have created https://github.com/eclipse/smarthome/pull/2451 as a fix. It should be available by the end of next week in the OH2 distro, watch out at https://community.openhab.org/c/openhab-2/news-important-changes where I’ll announce as soon as a new ESH update flows into OH2.
