Dtrace on the w3wp process on the EV Server shows:
{EVS} Started Converting EVS Search Input to SOAP Boolean text Query || User: XXXXX
{EVS} Exception: String was not recognized as a valid DateTime. Info:Unhandled Exception in Enterprise Vault Search. || User: XXXX || Diag: Type:System.FormatException ST: at System.DateTimeParse.Parse(String s, DateTimeFormatInfo dtfi, DateTimeStyles styles)| at Symantec.EV.Search.Server.Business.QueryElementProcessing.AnyQueryElementProcessor.ProcessDateQuery(QueryElement element)| at Symantec.EV.Search.Server.Business.QueryElementProcessing.AnyQueryElementProcessor.ProcessQuery(QueryElement element)| at Symantec.EV.Search.Server.Business.Service.BooleanTextQueryTransformer.Transform(SearchInput searchInputQuery)| at Symantec.EV.Search.Server.Business.Service.BooleanTextQueryTransformer.Transform(SearchQueryBase searchQuery)| at Symantec.EV.Search.Server.Business.ResultsProcessing.SOAPSearchRequestFormatter.Format(SearchInput searchInput)| at Symantec.EV.Search.Server.Business.Service.SOAPSearchHandler.GetSearchResults(SearchInput searchInput)| at Symantec.EV.Search.Server.Business.SearchOrchestrator.GetSearchResults(SearchInput searchInputs)| at Symantec.EV.Search.Server.Service.ContentManagementService.GetSearchResults(SearchInput searchInputs)| at Symantec.EV.Search.Client.Presentation.Model.SearchModel.GetResults(SearchInput searchInput)| at Symantec.EV.Search.Client.Presentation.Presenters.ResultsPresenter.LoadCalendarEvents(SearchInput searchInput, Filter resultsFilter)| at Symantec.EV.Search.Client.Host.Containers.CalendarContainer.LoadCalendar()| at Symantec.EV.Search.Client.Host.Containers.CalendarContainer.hiddenCalButton_clicked(Object sender, EventArgs args)| at Telerik.Web.UI.RadButton.OnClick(ButtonClickEventArgs e)| at Telerik.Web.UI.RadButton.RaisePostBackEvent(String eventArgument)| at Telerik.Web.UI.RadButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)| at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)| at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)| at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) Inner:None
The FormatException is thrown if the locale/culture on the Enterprise Vault (EV) server (where EVS site is hosted) is English (United States) and the short date format in date and time settings is changed to a format like dd/MM/yyyy or dd-MMM-yy (where Day dd comes before Month MM).
The date values in the search query are in format like MM/dd/yyyy, MM-dd-yyyy or MM.dd.yyyy i.e. Month comes before the day, only the separators can be different as per the EVS UI culture.

This issue has been addressed in version 14.1.1
Workaround
Apart from dd-MMM-yy or dd/MM/yyyy (formats where Day comes before Month), select any other short date format in Date and time formats on the EV server (where EVS is hosted).