Thank you for being the first ( that I know of ) who tested the upgrade in an active setup 
I will try to deliver a patch for that issue
Hello,
I’ve also updated to Owncloud 10 and I think I have the same problem now:
{"reqId":"PXQdwejGgImWYBSW7zTS","level":3,"time":"2017-11-08T18:13:10+00:00","remoteAddr":"172.17.42.1","user":"Administrator","app":"index","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"Exception: {\"Exception\":\"OCP\\\\Files\\\\NotPermittedException\",\"Message\":\"No create permission for folder \\\/avatars\\\/7b\",\"Code\":0,\"Trace\":\"#0 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AvatarManager.php(99): OC\\\\Files\\\\Node\\\\Folder->newFolder('7b')\\n#1 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AvatarManager.php(120): OC\\\\AvatarManager->getFolder(Object(OC\\\\Files\\\\Node\\\\Folder), '7b')\\n#2 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AvatarManager.php(91): OC\\\\AvatarManager->getAvatarFolder(Object(OC\\\\User\\\\User))\\n#3 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/TemplateLayout.php(101): OC\\\\AvatarManager->getAvatar('Administrator')\\n#4 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/legacy\\\/template.php(232): OC\\\\TemplateLayout->__construct('user', 'files')\\n#5 \\\/var\\\/www\\\/owncloud\\\/lib\\\/public\\\/AppFramework\\\/Http\\\/TemplateResponse.php(156): OC_Template->fetchPage()\\n#6 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AppFramework\\\/Http\\\/Dispatcher.php(112): OCP\\\\AppFramework\\\\Http\\\\TemplateResponse->render()\\n#7 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AppFramework\\\/App.php(98): OC\\\\AppFramework\\\\Http\\\\Dispatcher->dispatch(Object(OCA\\\\Files\\\\Controller\\\\ViewController), 'index')\\n#8 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/AppFramework\\\/Routing\\\/RouteActionHandler.php(46): OC\\\\AppFramework\\\\App::main('ViewController', 'index', Object(OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer), Array)\\n#9 [internal function]: OC\\\\AppFramework\\\\Routing\\\\RouteActionHandler->__invoke(Array)\\n#10 \\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/Route\\\/Router.php(307): call_user_func(Object(OC\\\\AppFramework\\\\Routing\\\\RouteActionHandler), Array)\\n#11 \\\/var\\\/www\\\/owncloud\\\/lib\\\/base.php(928): OC\\\\Route\\\\Router->match('\\\/apps\\\/files\\\/')\\n#12 \\\/var\\\/www\\\/owncloud\\\/index.php(56): OC::handleRequest()\\n#13 {main}\",\"File\":\"\\\/var\\\/www\\\/owncloud\\\/lib\\\/private\\\/Files\\\/Node\\\/Folder.php\",\"Line\":162}"}
I’ve corrected the permissions like @miclan but I still can’t login.
EDIT: Ive tested it. I can access the folder avatars as user www-data via shell.
If I create the requested subfolders of avatars manually as www-data the I can login.
EDIT: Btw. where is the data now stored? Before the upgrade the data was on an separate volume, but now it is somewhere else on the root volume.
EDIT2: I still use UCS 4.1
I’ve found a solution (last post):