I have fixed the problem by updating the capybara gem to the latest version at present (3.33.0), before that, I had been testing with the version 3.10.1. After the update, the previous error has no longer happened and when I have forced an error in the test, I have recived a correct error message from Capybara:
Capybara::ExpectationNotMet: expected to find css ".detailtable" within #<Capybara::Node::Element tag="div" path="/HTML/BODY[1]/DIV[1]/DIV[2]/DIV[3]/DIV[1]"> at least 1 time but there were no matches
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