Hi @Eirik_Hanssen,
I recently posted the same observation about access being denied for an author upload of a revision file (Authors can't upload revised review file OJS 3.2). I’m aware of the workaround mentioned in a reply, whereby the author uses the Discussions tool and attaches the file there. But, in my post, I mentioned my preferred workaround, which is to make sure that another editorial decision is entered with the box checked for there not being another round of review. It doesn’t matter if there actually is another round. I also enter the decision without sending an email so as to not confuse the author. HOWEVER, there appears to be a problem with the software that should be fixed. No such workarounds should be needed.