Login loop of WebClient occur if accessed from WAN

I’m not expert of Nextcloud. I installed nextcloud 3 month ago, and have just upgraded. I’m troubled because login loop occur because this issue is appear if i only access from WAN. If i access to Nextcloud from private network, it works with no problem.

I already checked /path to nextcloud/data/nextcloud.log, however, there is no lines of level 3(Error). So, I tried with changing log level 1(Debug), then i get some lines as below…

{"reqId":"SWlQZQ6INa4XIQWY98qz","level":0,"time":"2018-12-10T18:42:33+09:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--",
"app":"core","method":"GET","url":"\/nextcloud\/index.php\/apps\/files\/","message":{"Exception":"OC\\AppFramework\\Midd
leware\\Security\\Exceptions\\NotLoggedInException","Message":"Current user is not logged in","Code":401,"Trace":[{"file
":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Middleware\/MiddlewareDispatcher.php","line":95,"function":"before
Controller","class":"OC\\AppFramework\\Middleware\\Security\\SecurityMiddleware","type":"->","args":[{"__class__":"OCA\\
Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatch
er.php","line":98,"function":"beforeController","class":"OC\\AppFramework\\Middleware\\MiddlewareDispatcher","type":"->"
,"args":[{"__class__":"OCA\\Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/nextcloud\/lib\/private\/
AppFramework\/App.php","line":118,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":
[{"__class__":"OCA\\Files\\Controller\\ViewController"},"index"]},{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFrame
work\/Routing\/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["
ViewController","index",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"_route":"files.view.index"}
]},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->","args":[{"_route":"files.v
iew.index"}]},{"file":"\/var\/www\/nextcloud\/lib\/private\/Route\/Router.php","line":297,"function":"call_user_func","a
rgs":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},{"_route":"files.view.index"}]},{"file":"\/var\/www\
/nextcloud\/lib\/base.php","line":987,"function":"match","class":"OC\\Route\\Router","type":"->","args":["\/apps\/files\
/"]},{"file":"\/var\/www\/nextcloud\/index.php","line":42,"function":"handleRequest","class":"OC","type":"::","args":[]}
],"File":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Middleware\/Security\/SecurityMiddleware.php","Line":143,"C
ustomMessage":"--"},"userAgent":"Mozilla\/5.0 (Macintosh; Intel Mac OS X 10_14_1) AppleWebKit\/537.36 (KHTML, like Gecko
) Chrome\/70.0.3538.113 Safari\/537.36 Vivaldi\/2.1.1337.51","version":"14.0.4.2"}

Does anyone have any solution?

Nextcloud version : 14.0.4.2
OS version : Ubuntu 18.04.1 LTS
Apache version : Apache/2.4.29 (Ubuntu)
PHP version : PHP 7.2.10-0ubuntu0.18.04.1

This issue is not occur when Nextcloud ver.13(I don’t remember minor version), and start from upgraded.