Hello! Yes, you were right 
The problem was caused by the Custom Trust Level plugin. Our programmer did as you and @pfaffman advised, very grateful to both of you for the excellent support 
Yes, you may be right, I understand that there are too many plugins and in the future (during any updates) errors may occur. But, I’m just starting a new project with AI and I want to test all the plugins in order to understand which of them will be really useful for my project, and which ones I won’t need later.