The exact error, which occurs when pressing Ctrl + ’ for the API reference, is:
GLib.GException: The location or file could not be found.
at Gnome.Url.Show (System.String url) [0x00000] in :0
at MonoDevelop.Platform.GnomePlatform.ShowUrl (System.String url) [0x00000] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/addins/GnomePlatform/GnomePlatform.cs:134
at MonoDevelop.Ide.DesktopService.ShowUrl (System.String url) [0x00000] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/DesktopService.cs:94
at Unity.Utilities.SearchReferenceCommandHandler.Run () [0x00000] in :0
at MonoDevelop.Components.Commands.CommandHandler.Run (System.Object dataItem) [0x00000] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.Commands/CommandHandler.cs:61
at MonoDevelop.Components.Commands.CommandHandler.InternalRun (System.Object dataItem) [0x00000] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.Commands/CommandHandler.cs:42
at MonoDevelop.Components.Commands.CommandManager.DefaultDispatchCommand (MonoDevelop.Components.Commands.ActionCommand cmd, MonoDevelop.Components.Commands.CommandInfo info, System.Object dataItem, System.Object target, CommandSource source) [0x00064] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.Commands/CommandManager.cs:1096
at MonoDevelop.Components.Commands.CommandManager.DispatchCommand (System.Object commandId, System.Object dataItem, System.Object initialTarget, CommandSource source) [0x002d1] in /home/builduser/buildslave/monodevelop/build/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.Commands/CommandManager.cs:1067