The problem is in FILES application itself. In my opinoin,this is probably due to bug in Files aplication uninstalling process. In fact, old version Files aplication folder is not deleted, while new one is installed. (I tried to reinstall Files several times, and as I noticed later, in Application folder there were as many Files intallation folders as I Tried to reinstall). I was forced to solve it several times, in fact every time Files new version was out. As far as I know, there are two solutions:
First one (and the only, if you running non jailbroken device): run iPhone "restore to factory settings" in iTunes or run "reset all content" right on iPhone (with NO RESTORE FROM BACKUP after that).
Second one (jailbreak device required): in iPhone file browser (i-FunBox, iPhone Explorer etc.) go to USER - APPLICATIONS an find (folders are named by 32 digit number code) all "FILES" instalation folders and simply delete them. Restart iPhone and install new version of Files. For me always worked flawless this way.
Hope this will help...