I am having the same issue, my Apache log shows as follows:
[Wed Apr 27 11:47:10.870938 2016] [access_compat:error] [pid 3572] [client 10.57.2.57:3873] AH01797: client denied by server configuration: /var/www/dokuwiki/data/security.png, referer: https://10.57.2.36/start?do=admin
[Wed Apr 27 11:47:11.214984 2016] [access_compat:error] [pid 3573] [client 10.57.2.57:3871] AH01797: client denied by server configuration: /var/www/dokuwiki/data/security.png, referer: https://10.57.2.36/start?do=admin
[Wed Apr 27 11:48:25.171381 2016] [access_compat:error] [pid 3572] [client 10.57.2.57:3909] AH01797: client denied by server configuration: /var/www/dokuwiki/data/security.png, referer: https://10.57.2.36/start?do=admin
[Wed Apr 27 11:48:25.741111 2016] [access_compat:error] [pid 3573] [client 10.57.2.57:3908] AH01797: client denied by server configuration: /var/www/dokuwiki/data/security.png, referer: https://10.57.2.36/start?do=admin
[Wed Apr 27 11:34:25.548463 2016] [:error] [pid 3574] [client 10.57.2.53:63042] PHP Fatal error: Uncaught Error: Call to undefined function utf8_decode() in /var/www/dokuwiki/inc/utf8.php:152\nStack trace:\n#0 /var/www/dokuwiki/inc/utf8.php(402): utf8_strlen('user')\n#1 /var/www/dokuwiki/inc/common.php(1085): utf8_ucfirst('user')\n#2 /var/www/dokuwiki/lib/plugins/userhomepage/action.php(362): parsePageTemplate(Array)\n#3 /var/www/dokuwiki/lib/plugins/userhomepage/action.php(137): action_plugin_userhomepage->applyTemplate('public')\n#4 /var/www/dokuwiki/inc/events.php(211): action_plugin_userhomepage->redirect(Object(Doku_Event), Array)\n#5 /var/www/dokuwiki/inc/events.php(70): Doku_Event_Handler->process_event(Object(Doku_Event), 'BEFORE')\n#6 /var/www/dokuwiki/inc/actions.php(32): Doku_Event->advise_before()\n#7 /var/www/dokuwiki/doku.php(124): act_dispatch()\n#8 {main}\n thrown in /var/www/dokuwiki/inc/utf8.php on line 152
Any help?