Unable to add more pluggins to my mix

Good morning guys
i am almost at the end of a mix .I would like to add more plugins to backing vocals unfortunately every time i try adding or removing a plugin i get an error message asking me to contact support.Please help:(
I’m using cubase 5 . 64 bit . processor i7

Hi and welcome,

Do you use Cubase 5 or Cubase 9.5, please?

What kind of plug-ins do you have in your project? Especially on Windows, there is limit, based on Microsoft restriction. It’s not a specific count, it depends on the concrete plug-ins.