I agree with resolving files issues during upgrade, but I’m afraid that in my case there’s some inconsistent data in our database 
I remember another problem (https://forum.pkp.sfu.ca/t/tasks-not-loading-for-certain-users/32074/6) with “permanent loading”. Now I checked the solution proposed then and found 3 submissions without section_id. I corrected this in the database. But this did not change anything - as before, nothing is working inside the system