Data is Null

The SharePoint item being crawled returned an error when requesting data from the web service. ( Error from SharePoint site: Data is Null. This method or property cannot be called on Null values. )

I found the code:
web.SiteGroups.Add(groupName, owner, null, null);

Solution:

It seems that the "default user" parameter is not the culprit here after all, but the fourth parameter is the one that actually causes this error. This is a description parameter, and even an empty string is OK. See this Microsoft Support article http://support.microsoft.com/kb/2323206

Form Template's status remains forever in 'installing' or 'deleting' state

Form template’s status never leaves “Installing…”, “Deleting…”, “Upgrading…”  or “Removing…” states
These states seem to indicate that you're running a multiple-machine server farm, and on your server farm, you're running into some issues in propagating changes to all of the machines.  If you have not done so already, I highly recommend turning on the following services on each machine: SPAdmin, and SPTimerV3 You can do this by running:

                net start SPTimerV3
                net start SPAdmin

On each machine (If you are doing this on a SharePoint Farm , you want to make sure the Timer Service is running on all the Servers on the Farm).  Net start is a ensure semantic, so this will not inadvertently toggle or cause any damage if run on a machine where the service is already started.  Now that that's done, you can go on to correcting the problems that you have.
From Central Administration, go to the Operations page, under the Global configuration group, click on Timer Job Status.  On that page, look for timer jobs that have the name in the following formatting.  If you filename is FOO.xsn, it will look like:
Windows SharePoint Services Solution Deployment for "form-FOO.wsp"
See if there was a failure.  If so, go back a page, and go to Timer Job Definitions.  Drill down in the timer job definition that you care about and you can perform the following:

Generally:

1. Try to restart the job if that is available.
2. If restart is not available, delete the job, then attempt your initial action again .

SharePoint shows no help

Problem: Click on the help icon in SharePoint, it opens a window but shows no help text.
Instead you see messages like:

-No Help Collection was found.
-Help content cannot be displayed. (Or similar in your language)

Solution:
Then you have to follow this resolution: http://support.microsoft.com/kb/939313/en-us

When you run the commandline (execution may take 10-15 min.)
hcinstal.exe /act InstallAllHCs
and you experience the error
Outcome code is: 256
you have run this code probably with not sufficient privileges to the database. Run this code with your farm admin account.

Unable to deactivate and delete sandbox solution?

1. I have made a template from a given SharePoint site
2. Uploaded it to another site collection to create new sites. (So the web site template is actually a sandboxed solution.)
3. Now, I created a subsite from that template, deleted that site and then wanted to remove the solution. Therefore I had to deactivate it before.
4. Tried to deactivate the solution - Boooom...

The error message I received was:

"Unable to access web scoped feature (Id: 10ef0abc-dbd4-484a-b9a9-4cc425e7dc0b) because it references a non-existent or broken web (Id: 56ab1e1e-a09a-4783-9947-3d79bc6473aa) on site 'http://wfe2'. Exception: System.ArgumentException: Value does not fall within the expected range. at Microsoft.SharePoint.SPWebCollection.get_Item(Guid id) at Microsoft.SharePoint.SPFeatureEnumeratorBase.GetCachedWeb(SPSite site, Guid webId, Guid featureId)"

The solution is, there is still the deleted site in the SharePoint database and the features from within the template are activated. Sandboxed solutions containing site templates cannot be deactivated and deleted when there are still sites available based on this template. So the features are a reference to that template (and its solution) and you are not able to deactivate it as long as this reference exists.

You have to don't just delete the site, you have to remove it from the recycle bin as well. So you have to be a site collection admin and delete the site from "site collection recycle bin".

SQL Server 2008 datetime2 issue

If you got the following error in an INSERT statment from entity framework to MSSQL 2008: System.Data.SqlClient.SqlException: The conversion of a datetime2 data type to a datetime data type resulted in an out-of-range value.

Solution:
The problem occurrs when you use Entity Framework to INSERT a System.DateTime (a not initialized one like 0001-01-01 00:00:00.0000000) into the SQL2008 DB. Change the datatype in DB to datetime2 and everything will run smoothly.
Another solution is to initialize the datetime field with a value greater than 1/1/1753.

Data Source Conflict

If you are developing BCS solutions in Visual Studio, you might have experienced the following error when you attempt to create or update a new item in an external list:

DataFormWebPartException: Data Source Conflict: Your changes conflict with those made concurrently by another user. Refresh the form and submit your changes again.

Solution: In your Visual Studio 2010 BCS project, find the Create or Update method that's failing. Verify that each of the fields in your input type descriptor have the Creator property (for the Create method) or Updater property (for the Update method) set to True.
Note, this property should not be set to true for the identifier field if the LOB auto generates the value. You also need to delete and re-add the external list before the changes take effect. (Because the object definition of the external content type is saved in the existing list and does not change when you update the BCS model.)

There are no addresses available for this application

After creating a Business Data Connectivity Service Application in SharePoint 2010 I wasn’t able to access the service application with the BDC models, external systems and external content types.
I tried to deploy a Visual Studio Solution by hitting F5. The error message I received was: "Error occurred in deployment step 'Add Solution': There are no addresses available for this application.” Clicking the BCS under "Manage service applications" got the same error.
Solution:
Check that the service under "Manage services on server" is running. If the error occures despite the service is running, restart the IIS with "iisreset".