Hi,
I'm not supplied by E.On or E.On Next myself, so I'm not really able to test this out for myself. If Landmark stops by, he might be able to give this a go though.
With that being said, I just made a single attempt to use login details that I know are completely impossible (don't worry, I didn't do it more than once) and the system immediately and correctly rejected them. I also didn't notice any issues with the button, but that might just be because it behaves differently when it accepts your login details, against when it spits them straight out.
Given that you've tested this using browsers based on both the Chromium engine (which is what Edge and Google Chrome are based on) and WebKit engine (used by Safari), the next logical step I can think of would be to try using Firefox on your PC for a bit, since it's based on the Firefox Quantum engine. You can download it from the Mozilla website at
https://www.mozilla.org/en-GB/firefox/ . This is just to see if the issue affects Firefox as well, so you can uninstall it when you're done if you'd rather not keep it around (go into Start > Settings > Apps > Apps & Features > locate Firefox > click the three dots to the right > Uninstall > follow the prompts). The Mozilla Maintenance Service will also uninstall itself automatically, usually within five minutes after Firefox is removed as long as you don't have any other Mozilla stuff installed, so you don't need to worry about that.
If you're still having the same issue in Firefox as well, then that suggests it's a server side issue and I can ask Hannah or Beki to see if they can flag it up.