wp customize broke after updating plugins, where to dig?

Question

WordPress

First I updated wp itself (everything was fine and worked), then a pack of all plugins, after updating them, wp customize broke and now it looks
like

this
:

spoiler

• Add Meta Tags
  
• Custom Permalinks
  
• XML-RPC
  
• Duplicate Post
  
• Email Subscriber
  
• English WordPress Admin
  
• Grid Columns
  
• Huge IT Image Gallery
  
• image cleaner
  
• Image Regenerate & Select Crop
  
• Imsanity
  
• List category posts
  
• Manage notification emails
  
• MaxButtons
  
• Menu Icons
  
• Responsify WP
  
• Restrict Widgets
  
• Sangar Slider Lite
  
• Short URL
  
• Slate Admin Theme
  
• Social Login
  
• The Events Calendar
  
• TinyMCE Advanced
  
• Velvet Blues Update URLs
  
• WordPress Importer
  
• WP Category Permalink
  
• WP Change Default Email
  
• WP Mail SMTP
  
• WP Subscribe
  
• WP Super Cache
  
• Yandex.News Feed by Teplitsa
  
• Массовое удаление
  
• Переводчик Loco
  
• Проверка нерабочих ссылок
  

PS ... I apologize in advance if I duplicated a stupid question that has an answer. Googling, alas, was not strong enough.

Answer 1

0 make a backup of the database and files
1 disable all plugins and see if the customizer works
...
depending on the answer to the first paragraph, it will be clearer what to do next