Problem description: You are prompted to enter your credentials each time you open a Microsoft Office document stored in a SharePoint document library
Each time you open a Microsoft Office document, you are prompted to enter your credentials again, even if you check the box remember my password. For non Microsoft Office documents, such as PDF files, you are not prompted to enter your credentials. The credentials prompt is not taken into account apparently: if you choose Cancel in the prompted window without entering your credentials, the Microsoft Office document is still opened.
Solution: Modify the registry
There is a fix for Microsoft Vista: you should have service pack 1 installed of Microsoft Vista as the hotfix is automatically included in service pack 1.
The next step is to modify your registry as described in the Microsoft KB You are prompted to enter your credentials when you access an FQDN site by using a Windows Vista-based client computer that has no proxy configured.
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