I was able to solve my problem six months ago with the general recommendation to track down incompatibilities of plugins: Deactivate all other plugins one after the other on a staging site or at night in maintenance mode until the popups work again. For me, Popup Maker works fine with the current WordPress version. As described above, the problem was with a different plugin. The Popup Maker support team has no way of knowing and cannot figure out which combination of plugins resulted in an error. By the way, it is often the caching plugin that cause unwanted side effects.