Error and Solution

I struggled a lot with this problem and thought I'd post the solution so you may not have to spends days on the solution as I did.

I had a VS 2003 web project and was using DevExpress 6.3.6, but the version does not matter so much in this case.

The web project ran fine on my developer machine, but when deployed to the test server (maintained but someone else) it threw this error:
Derived method 'get_IsEmpty' in type 'DevExpress.Web.ASPxDataControls.LookAndFeelStyle' from assembly 'DevExpress.Web.ASPxDataControls.v6.3, Version=6.3.6.0, Culture=neutral, PublicKeyToken=79868b8147b5eae4' cannot reduce access.

I won't go into details what I tried to resolve this but want to mention that it included setting a a new developer's workstation from scratch!

The solution was so simple: the web site on the test server was configured to use .NET 2.0 instead of .NET 1.1! The two versions seem to be compatible enough to not throw an error in many cases, but only for certain pages.

I had seen that when you run a .NET 2.0 application on a virtual directory configured to use .NET 1.1 ti would show an error that indicates this, so I had assumed that it would do the same in the reverse case, which is the reason why I failed to look into this earlier.

May this information save some time for someone else.