Hello James,
In such scenarios (i.e. when a page with our components is rendered in IE compatibility mode), we recommend switching to the corresponding compatibility mode using the ASPxWebControl.SetIECompatibilityMode method. See the following KB article:
How to adjust DevExpress ASP.NET controls when opening a page in the Internet Explorer Compatibility Mode
If this solution doesn't help you, provide me with a sample project that I can use to replicate this error on my side. Also, specify the IE version that you are using.
It will help me better understand the cause of the issue and provide you with the most suitable solution.
I am looking forward to your feedback.