I had this error too when upgrading to 5.4. It happens with sites that have a default localization other than en-US. I had quite some trouble getting logged in as host (I don't have administrator rights to that portal). Once admin, goto site settings. New in 5.4 is the possibility to give different seetings On top of the site settings page, you will see a combobox "language settings".

Check the basic settings for each locale: you will see that the information got a little messed up un the upgrade. Once I had restored the correct values, the problem no longer occurred.

I was lucky I was able to log in as host. My host portal has en-US as default settings, and from there I could log in.

If you're not able to log in, I would suggest you restore 5.2.2, enable en-US and set this as default language. Then perform the upgrade, correct the basic settings of all your locales, return the default locale to its original settings and disable en-US. I have not tried this myself, but for what I understand of this bug, I believe this might very well be a workable work-around.

Once you're in the 5.4, this will no longer be an issue.

Success!

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The one-eyed in the land of the blind.