Regarding the "System.Windows.Data Error: 26" we tried to reproduce the error on our side but to no avail.
Could you give us more information about how that error is reproduced. Additional information on the use case of the RadWatermarkTextBox, like where is it placed, does it have customizations, bindings etc. would be very helpful. I recorded and attached a short video showing how the scenario run on our side, please let us know if we missed something.
The "System.Windows.Data Error: 4" is a common error caused by the way some controls are loaded by the Framework. Some of the bindings are broken at the start of the application but fast after that they are resolved and no critical exception is thrown. You shouldn't worry about this kind of errors.
Suppressing them is possible by setting a different Trace level in your application as described in this
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.