Here is the stack trace:

at Umbraco.Core.Mandate.That[TException](Boolean condition, Func`1 defer) in d:\Dev\Umbraco7\src\Umbraco.Core\Mandate.cs:line 98
   at Umbraco.Core.Models.ContentBase..ctor(String name, Int32 parentId, IContentTypeComposition contentType, PropertyCollection properties) in d:\Dev\Umbraco7\src\Umbraco.Core\Models\ContentBase.cs:line 44
   at Umbraco.Core.Models.Content..ctor(String name, Int32 parentId, IContentType contentType, PropertyCollection properties) in d:\Dev\Umbraco7\src\Umbraco.Core\Models\Content.cs:line 71
   at Umbraco.Core.Models.Content..ctor(String name, Int32 parentId, IContentType contentType) in d:\Dev\Umbraco7\src\Umbraco.Core\Models\Content.cs:line 56
   at Umbraco.Core.Persistence.Factories.ContentFactory.BuildEntity(DocumentDto dto) in d:\Dev\Umbraco7\src\Umbraco.Core\Persistence\Factories\ContentFactory.cs:line 32
   at Umbraco.Core.Persistence.Repositories.ContentRepository.CreateContentFromDto(DocumentDto dto, Guid versionId) in d:\Dev\Umbraco7\src\Umbraco.Core\Persistence\Repositories\ContentRepository.cs:line 593
   at Umbraco.Core.Persistence.Repositories.ContentRepository.PerformGet(Int32 id) in d:\Dev\Umbraco7\src\Umbraco.Core\Persistence\Repositories\ContentRepository.cs:line 75
   at Umbraco.Core.Persistence.Repositories.RepositoryBase`2.Get(TId id) in d:\Dev\Umbraco7\src\Umbraco.Core\Persistence\Repositories\RepositoryBase.cs:line 97
   at Umbraco.Core.Persistence.Repositories.ContentRepository.d__24.MoveNext() in d:\Dev\Umbraco7\src\Umbraco.Core\Persistence\Repositories\ContentRepository.cs:line 113
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source)
   at Umbraco.Web.Trees.ContentTreeControllerBase.GetTreeNodes(String id, FormDataCollection queryStrings)
   at Umbraco.Web.Trees.TreeControllerBase.GetNodes(String id, FormDataCollection queryStrings)
   at Umbraco.Web.Trees.ApplicationTreeExtensions.TryLoadFromControllerTree(ApplicationTree appTree, String id, FormDataCollection formCollection, HttpControllerContext controllerContext)
   at Umbraco.Web.Trees.ApplicationTreeController.d__17.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Umbraco.Web.Trees.ApplicationTreeController.d__4.MoveNext()

Well, that was dissapointing. First time I've posted in a forum for years and this is why, once again I resolved it myself with no help. This is something that I think Umbraco team need to look into however because I resolved it by deleteing the last nodes I created and all their dependencies but why would have creating some content through the CMS have caused this all on its own anyway? Here are the tables I had to delete from to get it working again.

DELETE FROM cmsPreviewXml WHERE nodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentVersion WHERE ContentId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsDocument WHERE nodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContent WHERE nodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsDocumentType WHERE templateNodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsTemplate WHERE nodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentType2ContentType WHERE parentContentTypeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentType2ContentType WHERE childContentTypeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentTypeAllowedContentType WHERE Id IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentTypeAllowedContentType WHERE AllowedId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM cmsContentType WHERE nodeId IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

DELETE FROM umbracoNode WHERE id IN (2163, 2164, 2166, 2167, 2168, 2169, 2170, 2171);

Thanks Matt, for the query! I've changed it somewhat and added the cmsPropertyData for version 7.1.4. Change the ids (or add / delete) for what nodes you want to delete.

BEGIN TRAN

DECLARE @Ids TABLE (
id int
)
INSERT INTO @Ids SELECT 2200
INSERT INTO @Ids SELECT 2201 

DELETE FROM cmsPreviewXml WHERE nodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentVersion WHERE ContentId IN (SELECT Id FROM @Ids);
DELETE FROM cmsDocument WHERE nodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContent WHERE nodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsDocumentType WHERE templateNodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsTemplate WHERE nodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentType2ContentType WHERE parentContentTypeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentType2ContentType WHERE childContentTypeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentTypeAllowedContentType WHERE Id IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentTypeAllowedContentType WHERE AllowedId IN (SELECT Id FROM @Ids);
DELETE FROM cmsContentType WHERE nodeId IN (SELECT Id FROM @Ids);
DELETE FROM cmsPropertyData WHERE contentNodeId IN (SELECT Id FROM @Ids);
DELETE FROM umbracoNode WHERE id IN (SELECT Id FROM @Ids);

COMMIT TRAN

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