To reproduce, create a free yahoo mail account. Using those credentials try this ruby code:

    require 'net/imap'
    Net::IMAP.debug = true
    conn = Net::IMAP.new('imap.mail.yahoo.com', 143, false)
    conn.instance_eval { send_command('ID ("GUID" "1")') }
    conn.authenticate('LOGIN', <username>, <password>)
    conn.select("INBOX")
    uids = conn.uid_search(['ALL'])
    conn.logout
    conn.disconnect

You should have at least one welcome email and the response from the conn.uid_search(['ALL']) will return "* SEARCH 1 \r\n" The trailing space before the CRLF seems unanticipated by search_response and causes the 'unexpected token' downstream in number. Here is my patch:

     def search_response # line 2706 imap.rb
        token = match(T_ATOM)
        name = token.value.upcase
        token = lookahead
        if token.symbol == T_SPACE
          shift_token
          data = []
          while true
            token = lookahead
#begin patch  - yahoo IMAP was returning pattern " SEARCH 1 2 \r\n  so was doing push on CRLF
=begin   before patch
            case token.symbol
              when T_CRLF
                break
              when T_SPACE
                shift_token
            end
            data.push(number)
=end
            case token.symbol
              when T_CRLF
                break
              when T_SPACE
                shift_token
              else
                data.push(number)
            end
#end patch
          end
        else
          data = []
        end
        return UntaggedResponse.new(name, data, @str)
      end

I confirmed this code is unchanged in p180 so didn't test it there. I hope this helps.