@MJB, see/stay on this other thread for your problem:Error when update to 3.1 version of ojs - #9 by MJB…
@Rondineli_Gama_Saad, did you get any warnings during the upgrade? – I think it could happen that some errors/warnings occurred but the updated run till the end and thus finished with that message…
Also, when you go to those pages that are blank now, do you see any error in your apache error log file?
Best,
Bozana