Removed the ContentPlaceHolder like you suggested - didn't help. This is what I have in the log:
2010-03-29 10:17:04,033 ERROR mojoPortal.Web.mojoBasePage - Error setting master page. Will try setting to default skin.en-US - 131.156.71.164
System.Web.HttpParseException: ContentPlaceHolder can only be used in .master files. ---> System.Web.HttpParseException: ContentPlaceHolder can only be used in .master files. ---> System.Web.HttpException: ContentPlaceHolder can only be used in .master files.
at System.Web.UI.WebControls.ContentPlaceHolderBuilder.Init(TemplateParser parser, ControlBuilder parentBuilder, Type type, String tagName, String ID, IDictionary attribs)
at System.Web.UI.ControlBuilder.CreateBuilderFromType(TemplateParser parser, ControlBuilder parentBuilder, Type type, String tagName, String id, IDictionary attribs, Int32 line, String sourceFileName)
at System.Web.UI.ControlBuilder.CreateChildBuilder(String filter, String tagName, IDictionary attribs, TemplateParser parser, ControlBuilder parentBuilder, String id, Int32 line, VirtualPath virtualPath, Type& childType, Boolean defaultProperty)
at System.Web.UI.TemplateParser.ProcessBeginTag(Match match, String inputText)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
--- End of inner exception stack trace ---
at System.Web.UI.TemplateParser.ProcessException(Exception ex)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding)
--- End of inner exception stack trace ---
at System.Web.UI.TemplateParser.ProcessException(Exception ex)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseFile(String physicalPath, VirtualPath virtualPath)
at System.Web.UI.TemplateParser.ParseInternal()
at System.Web.UI.TemplateParser.Parse()
at System.Web.Compilation.BaseTemplateBuildProvider.get_CodeCompilerType()
at System.Web.Compilation.BuildProvider.GetCompilerTypeFromBuildProvider(BuildProvider buildProvider)
at System.Web.Compilation.BuildProvidersCompiler.ProcessBuildProviders()
at System.Web.Compilation.BuildProvidersCompiler.PerformBuild()
at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)
at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile)
at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile)
at System.Web.UI.MasterPage.CreateMaster(TemplateControl owner, HttpContext context, VirtualPath masterPageFile, IDictionary contentTemplateCollection)
at System.Web.UI.Page.get_Master()
at mojoPortal.Web.mojoBasePage.SetupMasterPage()
at mojoPortal.Web.mojoBasePage.OnPreInit(EventArgs e)
2010-03-29 10:17:11,500 ERROR mojoPortal.Web.mojoBasePage - Error setting master page. Will try setting to default skin.en-US - 131.156.71.164
System.Web.HttpParseException: ContentPlaceHolder can only be used in .master files. ---> System.Web.HttpParseException: ContentPlaceHolder can only be used in .master files. ---> System.Web.HttpException: ContentPlaceHolder can only be used in .master files.
at System.Web.UI.WebControls.ContentPlaceHolderBuilder.Init(TemplateParser parser, ControlBuilder parentBuilder, Type type, String tagName, String ID, IDictionary attribs)
at System.Web.UI.ControlBuilder.CreateBuilderFromType(TemplateParser parser, ControlBuilder parentBuilder, Type type, String tagName, String id, IDictionary attribs, Int32 line, String sourceFileName)
at System.Web.UI.ControlBuilder.CreateChildBuilder(String filter, String tagName, IDictionary attribs, TemplateParser parser, ControlBuilder parentBuilder, String id, Int32 line, VirtualPath virtualPath, Type& childType, Boolean defaultProperty)
at System.Web.UI.TemplateParser.ProcessBeginTag(Match match, String inputText)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
--- End of inner exception stack trace ---
at System.Web.UI.TemplateParser.ProcessException(Exception ex)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding)
--- End of inner exception stack trace ---
at System.Web.UI.TemplateParser.ProcessException(Exception ex)
at System.Web.UI.TemplateParser.ParseStringInternal(String text, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding)
at System.Web.UI.TemplateParser.ParseFile(String physicalPath, VirtualPath virtualPath)
at System.Web.UI.TemplateParser.ParseInternal()
at System.Web.UI.TemplateParser.Parse()
at System.Web.Compilation.BaseTemplateBuildProvider.get_CodeCompilerType()
at System.Web.Compilation.BuildProvider.GetCompilerTypeFromBuildProvider(BuildProvider buildProvider)
at System.Web.Compilation.BuildProvidersCompiler.ProcessBuildProviders()
at System.Web.Compilation.BuildProvidersCompiler.PerformBuild()
at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)
at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile)
at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile)
at System.Web.UI.MasterPage.CreateMaster(TemplateControl owner, HttpContext context, VirtualPath masterPageFile, IDictionary contentTemplateCollection)
at System.Web.UI.Page.get_Master()
at mojoPortal.Web.mojoBasePage.SetupMasterPage()
at mojoPortal.Web.mojoBasePage.OnPreInit(EventArgs e)