Incorrect ApplicationMasterModule

At the customer I’m working a strange thing happened. In the intranet farm (4 WFE loadbalanced with NLB) on two of the WFE servers going to Site Actions > All Site Settings resulted in the following error:
File not found. at Microsoft.SharePoint.ApplicationRuntime.SPRequestModuleData.GetWebPartPageData(HttpContext context, String path, Boolean throwIfFileNotFound)
   at Microsoft.SharePoint.ApplicationRuntime.SPVirtualFile.GetVirtualPathProviderCacheKey(String virtualPath)
   at Microsoft.SharePoint.ApplicationRuntime.SPVirtualPathProvider.GetCacheKey(String virtualPath)
   at Microsoft.SharePoint.Publishing.Internal.CmsVirtualPathProvider.GetCacheKey(String virtualPath)
   [...]

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