Would love to know the full steps to repro this error so we can resolve this so other users don't have to worry about it, I tried this...

Install Auto DNN 4.9.5
Log in as Host
Admin -> Languages - install a language pack.
Admin -> Site Settings -> Advanced Settings -> Other Settings -> Make your new language the default
Admin -> Languages -> Disable en-US
Log out
Copy over latest version of DNN (5.4.2)
Run upgrade.
Log in
Click View, Edit, Layout on ribbon bar.

No errors.

Any better steps to repro?

Thanks...

Alex,

I think I misunderstod you:

You want me to make a new instal 4.9.5 (now where I'm running 5.4.2.), install the language and then upgrade to 5.4.2. again... ?
What will happend to the database?

Per

Thanks Alex :O)

I'm so nervous to move a comma now after have been missing my site for a whole week!

The solution Luiz came with, is not working for me:

First of all I see the Dotnetnuke.Httpmodules.dll in my BIN folder and when I try to put in the other code, it's coming an error...

I have deleted the content in the app_code HTML folder according to Sebastian in another thread, and this is working. If I try to add them again, I'm getting errors... (so this should be ok.
Can you imaging what's wrong, when everything is working perfect when logged in, but not when I'm logged out:
www.spaininfogroupproperties.com/prop

Regards,
Per