Errors while updating from EPiServer 7.0 to 7.1

Errors when updating from EPiServer 7.0 to 7.1 in the Add on store

Error #1

You update by using EPiServers built in Add-on system and suddenly the Editmode tab and possibly the Add-on tab disappears.

The problem is that the update removes the old add-ons and then crashes with no rollback because it can't overwrite files in the modules and modulesbin folders in the site.

To repair this, copy the folders Modules and ModulesRepository (usually located in the vpp-folder) from a working site.
Then restart the site.

Now it works, but is still in the old version.

Ensure that the files in modules and modulesbin folders in the site are not write protected and accessible by the system.

Try to update through the add-on tab again.

Error #2

You update by using EPiServers built in Add-on system. The Editmode remains but the content of Edit mode is blank (white)

Try to clear the .net cache
It's usually stored in a folder that looks like this: "c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\"

Restart the site

Error #3

You update by using EPiServers built in Add-on system. And then the site responds with "An item with the same key has already been added."

You probably have the same dll in both the bin and modulesbin folder.

Remove the duplicate from the bin folder (probably EPiServer.Cms.Shell.UI.dll)