Hi Dennis,
If you are looking for a quick (dirty) fix:
The thing is mxui.js does a getAttribute on widgetId with uppercase 'i'. However, the attributes in the DOM are all lowercase and altough I think getAttribute is supposed to be case-insensitive, it only works when you use widgetid with lowercase 'i' in the getAttribute.
At least this worked for me as a temporary hotfix untill a cleaner solution or fix from Mendix comes available.
I have seen this same issue today when testing one of our widgets. Reproduced this in 6.10.10.
I have pinpointed the part of code that causes this problem in our source. Seems like Edge is doing some weird voodoo here, as it works in a normal fashion in other browsers.
I have submitted a ticket with Support and will see if R&D can fix this issue.
Edit (June 7): I have been informed that they have a fix for this, currently testing it. If all goes well, this will be fixed in 6.10.13 and 7.16 (upcoming release)
I now see your question, I have the same problem.
Also in free app https://userofpostalcodeho.mxapps.io/ in version 7.8
Not in the same tools rebuilt 7.0.2 version https://userofpostalcodeho101.mxapps.io/
Tried a different layout for every page, but same error.
Also see this problem at a customer in 7.4, same App in version 7.13.1 giving the same error, too.
Another customer on 7.13.1 is fine.
So it is not bug that every IE Edge user is experiencing, but certainly a lot.