A strange problem has come up in our DNN Evoq 8.5.0 installation (upgraded in the past from a v7 installation). When content managers are updating content on the site, occasionally a page that they're working on will completely disappear from the site. It last happened this morning after a content manager clicked Discard on a page she was editing. I have checked the recycle bin and database for any record of the page, and nothing is found. I see one possibly related error in the error logs (below). Any idea on how to stop this from happening? Thank you!
2017-11-06 09:45:46,846 [mtnpweb05][Thread:67][ERROR] DotNetNuke.Services.Exceptions.Exceptions - FriendlyMessage="Error: Text/HTML is currently unavailable." ctrl="ASP.desktopmodules_html_htmlmodule_ascx" exc="System.InvalidOperationException: Sequence contains more than one matching element
at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
at DotNetNuke.Professional.HtmlPro.Components.VersionController.GetVersion(Int32 moduleId, Int32 versionId)
at DotNetNuke.Modules.HtmlPro.HtmlModule.OnLoad(EventArgs e)"