What is the site?
It's a generic error message that is giving a likely cause for the login to fail on a pc. Most likely, the site doesn't support the safari browser on the ipad. The site might not support the ipad at all.
Try one of these browsers. Complain to the behind the times site.
Puffin
"Puffin Web Browser is a wicked fast Mobile Flash Browser. Once users experience the thrilling speed of Puffin, regular mobile Internet feels like torture. Puffin Free is the free version of the Puffin family and supports Adobe Flash over cloud during "off-peak hours" from 8AM to 4PM (subject to change without prior notice)"
Free:
https://itunes.apple.com/us/app/puffin-web-browser-free/id472937654?mt=8
Paid:
https://itunes.apple.com/us/app/puffin-web-browser/id406239138?mt=8
iPad web browsers set to IE. You can set an ipad web browser to clam it's a pc running IE:
http://www.macsinlaw.com/make-your-iphone-ipad-surf-like-internet-explorer-or-fi refox/
Photon Flash Player for iPad - Flash Video & Games plus Private Web Browser
https://itunes.apple.com/us/app/photon-flash-player-for-ipad/id430200224?mt=8
"java for iPad is now a reality! If you were ever wondering how to use java for iPad, now you can! With the Virtual-Browser for Firefox App, java for iPad is possible with one simple app download! Virtual-Browser for Firefox App opens up a wide variety of apps and games – like java for iPad – that you can use from your iPad"
http://www.virtualfirefox.com/java-for-ipad
By Splashtop Inc.
Flash Video Web Browser – Full Chrome, IE, Firefox, Safari Compatible
https://itunes.apple.com/us/app/flash-video-web-browser-full/id431331485?mt=8
VirtualBrowser for Firefox with Flash-browser, Java Player and Add-ons - iPhone Edition
https://itunes.apple.com/us/app/virtualbrowser-for-firefox/id609289360?mt=8