Object does not support property or method "attachEvent"
If we set the meta x-ua-compatible to ie=9 the error does not appear.
<meta http-equiv="X-UA-Compatible" content="IE=9">
attachEvent has been removed by IE11, this is a known issue and need to be fixed in a future version.
attachEvent issue seems not to be fixed completely in DNN 7.3.2
can you tell why this problem did not occur in DNNN 7.2.2? We did not have the issue there. Did you update Telerik?
DNN Corp is maintaining Telerik components by themselves, they applied IE11 fixes for DNN 7.3.2