Error Processing File Viewers / Unable to Format URL for page forum_thread_legacy values

I am seeing a number of new errors related to the processing and rendering of file viewers in 12.0.1.15778:

UnknownException: Error processing file viewers. (An unexpected error prevented your requested action from completing. The error has been logged for the administrator to review.) ---> System.Threading.ThreadAbortException: Thread was being aborted.
   at Telligent.Evolution.Components.FileViewerService.Render(Uri url, FileViewerViewType viewType, Int32& width, Int32& height, Boolean outputIsPersisted, Boolean limitToThemeMaximums)
   at Telligent.Evolution.Components.FileViewerService.Render(Uri url, FileViewerViewType viewType, Int32 width, Int32 height, Boolean outputIsPersisted, Boolean limitToThemeMaximums)
   at Telligent.Evolution.Api.Plugins.FileViewers.HtmlFileViewerPlugin.GetRenderedViewer(Uri url, FileViewerViewType viewType, Int32 width, Int32 height, FileViewerMediaType mediaType)
   at Telligent.Evolution.Api.Plugins.FileViewers.HtmlFileViewerPlugin.<>c__DisplayClass8_0.<ProcessFileViewers>b__0(Match x)
   at System.Text.RegularExpressions.RegexReplacement.Replace(MatchEvaluator evaluator, Regex regex, String input, Int32 count, Int32 startat)
   at System.Text.RegularExpressions.Regex.Replace(String input, MatchEvaluator evaluator)
   at Telligent.Evolution.Api.Plugins.FileViewers.HtmlFileViewerPlugin.ProcessFileViewers(String html, String target)

UnknownException: An error occurred while rendering a file viewer. [Url = {Forum Thread URL}]  See inner exception for details. (An unexpected error prevented your requested action from completing. The error has been logged for the administrator to review.) ---> System.Threading.ThreadAbortException: Thread was being aborted.
   at System.Threading.Thread.AbortInternal()
   at System.Threading.Thread.Abort(Object stateInfo)
   at System.Web.HttpResponse.AbortCurrentThread()
   at Telligent.Evolution.Forums.Plugins.Content.ForumApplicationType.ParseForumContext(PageContext context)
   at Telligent.Evolution.Forums.Plugins.Content.ForumApplicationType.ParseForumThreadContext(PageContext context)
   at Telligent.Evolution.Urls.Routing.UrlManager.ParseContext(RouteValueDictionary values)
   at Telligent.Evolution.Components.UrlFileViewer.RenderView(String url, FileViewerViewType viewType, Boolean isPersisted, Int32& width, Int32& height)
   at Telligent.Evolution.Components.UrlFileViewer.Render(Uri url, IFileViewerOptions options)
   at Telligent.Evolution.Components.FileViewerService.Render(Uri url, FileViewerViewType viewType, Int32& width, Int32& height, Boolean outputIsPersisted, Boolean limitToThemeMaximums)

InternalException: Unable to format url for page 'forum_thread_legacy' Values: {} (An internal issue occurred that prevented your action from completing successfully. Details have been logged for site administrators to resolve.)
   at Telligent.Evolution.Urls.Routing.UrlManager.<>c__DisplayClass15_0.<FormatUrl>b__0()
   at Telligent.Evolution.Caching.Implementations.CacheService.Get[T](String key, Func`1 load, CacheScope scope, Func`1 tags, Nullable`1 expiresAfter, Object syncObj)
   at Telligent.Evolution.Urls.Routing.UrlManager.FormatUrl(String pageName, Dictionary`2 parms, RouteCollection routes)
   at Telligent.Evolution.Urls.Routing.UrlManager.FormatGroupUrl(String pageName, Group group, Dictionary`2 parms)
   at Telligent.Evolution.Api.Services.Url.BuildUrl(String urlName, Int32 groupId, Dictionary`2 parameters)

Interestingly to me is that they seem to come from paged content views more often than not and from tag pages. (The thread URL referenced in the second exception actually contains no media)

I am not sure if it matters, but the Site Theme Media Width and Height  fields for maximums have always been empty for us. Is it possible this is a null reference?

Added forum_thread_legacy values exception to report and updated title of post as it hadn't been clear that these three exceptions seem to occur as a trio.
[edited by: starksk at 1:52 PM (GMT 0) on Fri, May 21 2021]