I have had this happen twice now, I was working in document types adding new documents and adding new fields.  Then suddenly when clicking save I get a web config error.  Then every document type I click on a document I get a web config error.

I have read a few other people have had this problem but no one has given an easy fix.  I checked the error log and can see the following:

At /umbraco/create.aspx?nodeId=1096&nodeType=nodeTypes&nodeName=8Widget%20Master&rnd=30.9&rndo=31.6 (Referred by: http://master.i-pro1.co.uk/umbraco/create.aspx?nodeId=1096&nodeType=nodeTypes&nodeName=8Widget Master&rnd=30.9&rndo=31.6): System.NotSupportedException: The given path's format is not supported. at System.Security.Util.StringExpressionSet.CanonicalizePath(String path, Boolean needFullPath) at System.Security.Util.StringExpressionSet.CreateListFromExpressions(String[] str, Boolean needFullPath) at System.Security.Permissions.FileIOPermission.AddPathList(FileIOPermissionAccess access, AccessControlActions control, String[] pathListOrig, Boolean checkForDuplicates, Boolean needFullPath, Boolean copyPathList) at System.Security.Permissions.FileIOPermission.AddPathList(FileIOPermissionAccess access, String[] pathListOrig, Boolean checkForDuplicates, Boolean needFullPath, Boolean copyPathList) at System.Security.Permissions.FileIOPermission..ctor(FileIOPermissionAccess access, String path) at System.Web.Hosting.HostingEnvironment.MapPath(VirtualPath virtualPath) at umbraco.IO.IOHelper.MapPath(String path, Boolean useHttpContext) at umbraco.cms.businesslogic.template.Template.getMasterPageContent() at umbraco.cms.businesslogic.template.Template.setupNode() at umbraco.cms.businesslogic.template.Template.MakeNew(String Name, User u) at umbraco.nodetypeTasks.Save() at umbraco.presentation.create.dialogHandler_temp.Create(String NodeType, Int32 TypeId, Int32 NodeId, String Text) at umbraco.cms.presentation.create.controls.nodeType.sbmt_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

After clicking on the other document types, the error message remains the same except the id number changes.

Can someone please advise, I can restore the backup but if this happened on a live system the client would be extremely annoyed if I had to restore from the mornings backup when live data had already been entered.

Thanks