I made some actions: -downloaded bootstrap by hand and put it to vendor/assets/stylesheets/bootstrap (I created the bootstrap folder) -changed import "bootstrap" to import "bootstrap/bootstrap"
it worked, but I had objections if it is a good practise, so I made: -gem clean -gem update -bundle install -deleted the vendor/assets/stylesheets/bootstrap - everything what I put -changed back to import "bootstrap" and... it seems to work.