I updated to version 5.1 and now i'm getting this error as well.

AssemblyVersion: 5.1.0
PortalID: 4
PortalName: De Piaggio MP3 Club
UserID: 1
UserName: host
ActiveTabID: 111
ActiveTabName: Forum
RawURL: /dotnetnuke/Forum/tabid/111/ctl/Configuration/mid/433/language/en-US/Default.aspx
AbsoluteURL: /DotNetNuke/Default.aspx
AbsoluteURLReferrer: http://www.piaggio-mp3.com/dotnetnuke/Forum/tabid/111/ctl/ForumAdministration/mid/433/language/en-US/Default.aspx
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/530.5 (KHTML, like Gecko) Chrome/2.0.172.37 Safari/530.5
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 5e5d76ff-bb6c-4385-8838-241b1612e93f
InnerException: Not a legal OleAut date.
FileName: 
FileLineNumber: 0
FileColumnNumber: 0
Method: System.DateTime.DoubleDateToTicks
StackTrace: 
Message: System.ArgumentException: Not a legal OleAut date. at System.DateTime.DoubleDateToTicks(Double value) at DotNetNuke.Modules.Forum.Settings.Page_Load(Object sender, EventArgs e)
Source: 
Server Name: WIN64

I get the same error DNN 5.1.0 Forum 4.5.3

Message: System.ArgumentException: Inte ett giltigt OleAut-datum. vid System.DateTime.DoubleDateToTicks(Double value) vid DotNetNuke.Modules.Forum.ACP.General.Page_Load(Object sender, EventArgs e)
Source:
 

I hae only tested against DNN 4.9.4, and geting no issues on DateTime wit german language pack.  If I get the time I will try a test evironment on DNN 5.1.1 for testing.  It should be good to know what languages are used by the user.