I guess this might be the problem:
Studio one Official Anwer: https://support.presonus.com/hc/en-us/articles/210757986-Problem-with-songs-refusing-to-add-plug-ins-in-Studio-One-on-Windows
Cubase discussions:
Short: no solution. Even worse, plugins might not be loaded back into the project when a certain limit is reached. In my case I also have problems inserting any more vst instruments, not just insert plugins.
Doesn’t seem a problem on OS X, but switching is no solution either
cheers, lokotus