Using MahApps 1.4.1.0, an exception is thrown at app launch with this message :
Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception.
Seems to be thrown by MaterialDesignTheme.MahApps.Defaults.xaml
I'm having the same problem, For me the problem is
<ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.MahApps;component/Themes/MaterialDesignTheme.MahApps.Flyout.xaml" />
Removing that from my App.xaml lets my app start as usual

@bbougot @Insire Can one of you create a repo sample? Would be really useful. thx
https://github.com/Insire/InsireBot-V2 you can see the problem here
check my App.xaml Line 32 and uncomment that line and the error is back
edit: i updated https://github.com/Insire/MaterialDesignInXamlToolkitTestApp too, if you want a smaller test repo. I also noticed FlyoutAssist.HeaderColorMode and FlyoutAssist.HeaderShadowDepth missing. Not sure if related
Having this same issue.
Severity Code Description Project File Line Suppression State
Error An error occurred while finding the resource dictionary "pack://application:,,,/MaterialDesignThemes.MahApps;component/Themes/MaterialDesignTheme.MahApps.Defaults.xaml ".
When I look into the MaterialDesignThemes.MahApps assembly with dotPeek there is no type for FlyoutAssist, which exist in source code.
It seems like the 0.0.9 package includes the 0.0.1 assembly:

Should be verifiable by downloading the nuget package at https://www.nuget.org/packages/MaterialDesignThemes.MahApps/
Sorry guys...will try to look at this v. soon.
OK, please retry with https://www.nuget.org/packages/MaterialDesignThemes.MahApps/0.0.10
Most recent version works for me. Thanks for your work!
I am just experiencing the exact same error. Appeared using 0.0.11, tried 0.0.10 but still have the same issue.
Also having this issue with 0.0.11
I experience the same problem in this latest release 0.0.11 :(
The issue is because of the paket.lock file,
please check this:
https://github.com/ButchersBoy/MaterialDesignInXamlToolkit/issues/659
Most helpful comment
OK, please retry with https://www.nuget.org/packages/MaterialDesignThemes.MahApps/0.0.10