Well, as I use OBH (where I've never had any troubles with Autotimer, neither using OWIF nor the GUI, because it's built-in and you don't need to install it as external plugin) I tried to load BH that I keep in multiboot just for some tries.
I installed the plugin from the BH addons server and I actually got the same error.
I was able to import my current autotimers configured under OBH (using the autotimer.xml file) and I was also able to create one using the plugin GUI.
But all my autotimers, either the many imported or the new one created by the plugin, were not displayed by OWIF nor was possible to create a new one (when I tried to save it, nothing happened).
Notice that the plugin was also perfectly able to find the events contained in my autotimer.xml file.
So, it means there's a problem concerning only OWIF. Maybe you can open an issue in the Github forum.
Otherwise you can create and manage them only by the plugin GUI.