Please help us help you and fill where relevant:
Your LimeSurvey version: 6.3.1+231023 
Own server or LimeSurvey hosting: Own server
Survey theme/template: fruity_twentythree
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I just finished upgrading to version 6 from version 5 and I opened the first screen and none of the links work. I have to use the menu at the top of the screen to navigate.

I also tried opening a survey and got the following error messages404: Not Found The system is unable to find the requested action "undefined".The requested URL was not found on this server. If you entered the URL manually please check your spelling and try again.
If you think this is a server error, please contact Spec Surveys. #0 /home/hrf2iiwap6oo/public_html/vendor/yiisoft/yii/framework/web/CController.php(270): CController->missingAction('undefined')
#1 /home/hrf2iiwap6oo/public_html/application/controllers/LSBaseController.php(145): CController->run('undefined')
#2 /home/hrf2iiwap6oo/public_html/vendor/yiisoft/yii/framework/web/CWebApplication.php(282): LSBaseController->run('undefined')
#3 /home/hrf2iiwap6oo/public_html/vendor/yiisoft/yii/framework/web/CWebApplication.php(141): CWebApplication->runController('surveyAdministr...')
#4 /home/hrf2iiwap6oo/public_html/vendor/yiisoft/yii/framework/base/CApplication.php(185): CWebApplication->processRequest()
#5 /home/hrf2iiwap6oo/public_html/index.php(161): CApplication->run()
#6 {main} 

Also, when I go to "Statistics" (in Expert Mode) then click on Statistcs to export the statistics nothing happens.

Can someone point me in the right direction to get resolutions for these issues?

Thanks,
Vaughn