Do you get any errors in the log?

Only "Domain already exists in list (*)"

I'm not sure this is related to 'Indexes' (which is referring to Lucene/Examine)?

By this "Sometimes on our staging environment the Multi-Node Tree Pickers doesn't display any nodes anymore" do you mean in the back office?

You can rebuild the cache file by deleting the umbraco.config file and then restarting your app (bump web.config).

You can rebuild all Examine indexes by deleting the folder ~/App_Data/TEMP/ExamineIndexes and restarting your app - this will rebuild all indexes during startup.

I'd recommend trying one at a time so you can determine what the problem actually is. Also when these nodes don't display anymore, are you getting any JavaScript errors (you can see these in the dev console in your web browser)

Same behaviour when I deleted the umbraco.config and restarted pool.

After I deleted the ExamineIndexes and restarted the app - I get the following error when I save a node:

Server Error in '/' Application.

Value cannot be null.
Parameter name: attribute

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: attribute

Source Error: 

Stack Trace: 

[ArgumentNullException: Value cannot be null.
Parameter name: attribute]
   System.Xml.Linq.XAttribute.op_Explicit(XAttribute attribute) +150794
   UmbracoExamine.UmbracoContentIndexer.ReIndexNode(XElement node, String type) +190
   Examine.ExamineManager._ReIndexNode(XElement node, String type, IEnumerable`1 providers) +93
   umbraco.DocumentCacheEventHandler.Invoke(Document sender, DocumentCacheEventArgs e) +0
   umbraco.content.UpdateDocumentCache(Document d) +1032
   umbraco.library.UpdateDocumentCache(Document doc) +134
   umbraco.cms.presentation.editContent.Publish(Object sender, EventArgs e) +407
   System.EventHandler.Invoke(Object sender, EventArgs e) +0
   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +134
   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +204
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3804

Remark: The "Link to document" displays following error message: "Oops: this document is published but is not in the cache (internal error)"

And yes: The problem is in the back office but without any js errors.

What version are you using

umbraco v 4.11.5

I've seen that op_Explicit error before, it's to do with the content xml not containing the correct values. Here's a query with that text on our issue tracker:

http://issues.umbraco.org/issues?q=op_Explicit

Would be worth reading through those to see if any of those fixes relate to you.

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