Same issue here.
This is the error message that I see when I am looking for the username “pa” in the Share textbox.
{"reqId":"GbRsBwUz91FuUd2NI169","level":3,"time":"2017-08-13T06:27:43+00:00","remoteAddr":"1.2.3.4","user":"photos","app":"no app in context","method":"GET","url":"\/ocs\/v1.php\/apps\/files_sharing\/api\/v1\/sharees?format=json&search=pa&perPage=200&itemType=folder","message":"Exception: {\"Exception\":\"InvalidArgumentException\",\"Message\":\"Invalid cloud id\",\"Code\":0,\"Trace\":\"#0 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/apps\\\/files_sharing\\\/lib\\\/Controller\\\/ShareesAPIController.php(626): OC\\\\Federation\\\\CloudIdManager->resolveCloudId('pab@')\\n#1 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/apps\\\/files_sharing\\\/lib\\\/Controller\\\/ShareesAPIController.php(553): OCA\\\\Files_Sharing\\\\Controller\\\\ShareesAPIController->getEmail('pa')\\n#2 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/apps\\\/files_sharing\\\/lib\\\/Controller\\\/ShareesAPIController.php(492): OCA\\\\Files_Sharing\\\\Controller\\\\ShareesAPIController->searchSharees('pa', 'folder', Array, 1, 200, true)\\n#3 [internal function]: OCA\\\\Files_Sharing\\\\Controller\\\\ShareesAPIController->search('pa', 'folder', 1, 200, NULL, true)\\n#4 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/AppFramework\\\/Http\\\/Dispatcher.php(160): call_user_func_array(Array, Array)\\n#5 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/AppFramework\\\/Http\\\/Dispatcher.php(90): OC\\\\AppFramework\\\\Http\\\\Dispatcher->executeController(Object(OCA\\\\Files_Sharing\\\\Controller\\\\ShareesAPIController), 'search')\\n#6 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/AppFramework\\\/App.php(114): OC\\\\AppFramework\\\\Http\\\\Dispatcher->dispatch(Object(OCA\\\\Files_Sharing\\\\Controller\\\\ShareesAPIController), 'search')\\n#7 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/AppFramework\\\/Routing\\\/RouteActionHandler.php(47): OC\\\\AppFramework\\\\App::main('ShareesAPIContr...', 'search', Object(OC\\\\AppFramework\\\\DependencyInjection\\\\DIContainer), Array)\\n#8 [internal function]: OC\\\\AppFramework\\\\Routing\\\\RouteActionHandler->__invoke(Array)\\n#9 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/Route\\\/Router.php(299): call_user_func(Object(OC\\\\AppFramework\\\\Routing\\\\RouteActionHandler), Array)\\n#10 \\\/path\\\/to\\\/folder\\\/nextcloud\\\/ocs\\\/v1.php(77): OC\\\\Route\\\\Router->match('\\\/ocsapp\\\/apps\\\/fi...')\\n#11 {main}\",\"File\":\"\\\/path\\\/to\\\/folder\\\/nextcloud\\\/lib\\\/private\\\/Federation\\\/CloudIdManager.php\",\"Line\":68}","userAgent":"Mozilla\/5.0 (X11; Linux x86_64; rv:54.0) Gecko\/20100101 Firefox\/54.0","version":"12.0.1.5"}
As a consequence, I cannot share anything with use “pab” that does exist.