Application has broken up when I'm using your sample

[Englbach]

<TextBlock x:Name="txtContentMessage" Text="{Binding Text}" TextTrimming="CharacterEllipsis" 
                                       TextWrapping="Wrap" Foreground="{Binding Entities,ConverterParameter={Binding Text},Converter={StaticResource ChangedColorToUrlConverter},Mode=TwoWay}" MaxLines="3" Grid.Row="1"></TextBlock>

When I see your code:

<TextBlock x:Name="txtContentMessage" Text="{Binding Text}" TextTrimming="CharacterEllipsis" 
                                       TextWrapping="Wrap" MaxLines="3" Grid.Row="1">
                                    <m:MultiBindingHelper.MultiBindings>
                                        <m:MultiBindingCollection>
                                            <m:MultiBinding TargetProperty="Text">
                                                <m:Binding ConverterParameter="{Binding Text}" Converter="{StaticResource ChangedColorToUrlConverter}"></m:Binding>
                                                <m:Binding ConverterParameter="{Binding Entities}" Converter="{StaticResource ChangedColorToUrlConverter}"></m:Binding>
                                            </m:MultiBinding>
                                        </m:MultiBindingCollection>
                                    </m:MultiBindingHelper.MultiBindings>
                                </TextBlock>

And application has broken up when it was running.