I’m on Window 7 with Cubase Pro 10.0.40. I’m having the same issue.
Just to summarize for others after spending lots of time understanding this issue and how to workaround it.
The loading issue stems from a limitation on the number of loaded plugins. You can install a very small VST named “VST Checker” that will let you know how many slots you can further load at any moment. Usually, if you’re experiencing this issue, you’ll see that you have 0 additional plugins to load. Here’s the page (and thanks to TakashiWatanabe for coding it):
The best way to overcome it is by freezing some of your channels, to free up some slots for plugins loading. However, before freezing, you have to SOLO the channel. Otherwise, in many cases your Cubase might crash. This workaround was found by Otchenga Zoom in this page:
Good luck.