Getting "InternalException: .popup-border is undefined on line 5 in file /format-links-popup-panel.less", when trying to preview "Site" theme or widgets

Hello.

After upgrade to 10.1.5 - I cannot preview upgraded theme or widgets: pages in preview mode are broken and CssFileRenderingError error appears in the log.  

Any ideas?

<?xml version="1.0" encoding="UTF-8"?>

-<exceptions operatingSystem="Microsoft Windows NT 6.1.7601 Service Pack 1" machineName="XXXXXXXXXX" product="Telligent Community" version="10.1.5.9189" generated="2018-03-06T18:57:53.2844687Z"> 

<exception frequency="2" lastOcurred="2018-03-06T18:57:10.6770000Z" ipAddress="10.70.70.151" type="CssFileRenderingError" message=".popup-border is undefined on line 5 in file 'RnJkDDVn-UKHXci5kf5zmw/AAAAAAAAAAAAAAAAAAAAAA/3fc3f82483d14ec485ef92e206116d49/files/format-links-popup-panel.less?_t=636559594307454062': [4]: font-weight: @font-weight; [5]: .popup-border; -^ : background-color: @color-background;" httpReferrer="http://communitydev.exclusiveresorts.com:8111/" httpVerb="GET" httpPathAndQuery="/cfs-file/__key/themefiles/t-2100-h-000000000049a2c2-s-3fc3f82483d14ec485ef92e206116d49-stylesheetfiles-http-communitydevexclusiveresortscom-8111/screen.less?_t=636559594301360312" serverName="ZWEBDEV01" userAgent="Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_3) AppleWebKit/537." id="1"> InternalException: .popup-border is undefined on line 5 in file 'RnJkDDVn-UKHXci5kf5zmw/AAAAAAAAAAAAAAAAAAAAAA/3fc3f82483d14ec485ef92e206116d49/files/format-links-popup-panel.less?_t=636559594307454062': [4]: font-weight: @font-weight; [5]: .popup-border; -^ : background-color: @color-background; at Telligent.Evolution.Components.ThemeFiles.DotLessLogger.Log(String message) at dotless.Core.Loggers.Logger.Error(String message) at dotless.Core.LessEngine.TransformToCss(String source, String fileName) at dotless.Core.ParameterDecorator.TransformToCss(String source, String fileName) at Telligent.Evolution.Components.ThemeFiles.ProcessLessDirectivesInternal(Guid themeTypeId, Guid themeContextId, String themeName, String text, Boolean stage, Boolean attemptFix, Boolean& successful) at Telligent.Evolution.Components.ThemeFiles.ProcessLessDirectives(Guid themeTypeId, Guid themeContextId, String themeName, String text, Boolean stage, Boolean& successful) at Telligent.Evolution.Components.ThemeFiles.AdjustTextFile(Guid themeTypeId, Guid themeContextId, String themeName, String text, String fileName, Boolean stage, Boolean& successful) at Telligent.Evolution.Components.ThemeFiles.<>c__DisplayClass57_0.<AdjustCreateHostSpecificTextFile>b__0() at Telligent.Evolution.Components.DistributedLockService.Lock(String lockKey, Action action) at Telligent.Evolution.Components.ThemeFiles.AdjustCreateHostSpecificTextFile(String[] components, String& fileStoreKey, String& path, String& fileName) at Telligent.Evolution.Extensibility.Storage.Version1.CentralizedFileStorage.FindFile(String& fileStoreKey, String& path, String& fileName) at Telligent.Evolution.Extensibility.Storage.Version1.CentralizedFileStorage.TryGetFile(String fileStoreKey, String path, String fileName, ICentralizedFile& file) at Telligent.Evolution.Extensibility.Storage.Version1.CentralizedFileStorage.GetCentralizedFileByUrl(String url) at Telligent.Evolution.Components.FileHttpHandler.ProcessRequest(HttpContext context) at Telligent.Evolution.Urls.Routing.SiteUrlHttpHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) at System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error) at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb) at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus) at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) </exception>

 </exceptions>