29Consequently, since this is an action challenging the legality of a decision adopted under Article 4(3) of Regulation 2015/1589, without initiating the formal investigation procedure, it is, in principle, necessary to consider all the complaints and arguments raised by the applicant in the pleas in law relied on, in order to assess whether they make it possible to identify serious difficulties or doubts facing the Commission and requiring it to initiate the formal investigation procedure. In such an action, pleas challenging the compatibility of the aid are therefore to be assessed by the Court in the light of the existence of a serious difficulty, without its being necessary to declare them inadmissible (see judgment of 20 June 2019, a&o hostel and hotel Berlin v Commission, T‑578/17, not published, EU:T:2019:437, paragraphs 45 to 47 and 49 and the case-law cited; see also, to that effect, judgment of 24 May 2011, Commission v Kronoply and Kronotex, C‑83/09 P, EU:C:2011:341, paragraphs 57 and 58).