When using a browser, connect to PE5216/6216 can not login the error message is Login fail.  

Upgrade PE5216/6216 firmware to V1.5.145 or later to can solve this issue. And please adopt the attached WEB broswer to do so. (Firefox Portable V.107.0) 

You can also download for below web site:

https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./Mozilla%20Firefox%2C%20Portable%20Edition%20107.0/

To upgrade the firmware, please download the firmware and manual from 

https://www.aten.com/global/en/supportcenter/downloads/?q=PE5216

Please check the manual for how to upgrade firmware. 

If you still have problems, please contact ATEN Tech Support

https://eservice.aten.com/eServiceCx/Common/FAQ/askQuestion.do?type=None