Interface 'Psr\Container\ContainerInterface' not found trying to upgrade 8.3.2 to 8.5.4

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I'm trying to upgrade to 8.5.4 and have tried 3 methods, each ith the same result. Using the instructions inhttps://documentation.concrete5.org/developers/introduction/installa... I've tried
1. put the concrete5.x.x.x folder into your updates/ directory. the tell the dashboard to update
2. Replace the original concrete directory with the new one, empty the updates directory
and delete application/config/update.php file
In both cases I get the error Interface 'Psr\Container\ContainerInterface' not found with a display of some code concrete/vendor/container-interop/container-interop/src/Interop/Container/ContainerInterface.php showing that is is indeed trying to
use Psr\Container\ContainerInterface as PsrContainerInterface;


What is going wrong? How do I fix this?

There are a variety of reasons I'm not doing the upgrade completely through the dashboard, just one being that I'm wanting to do this for several sites all on the same server and don't want to keep having to download a file I've already got!

 
trentham replied on at Permalink Best Answer Reply
Whilst I never did find out what was going wrong here I have now found the doing the upgrade to 8.5.5 presents no such problems, whichever way I do the upgrade. I can only assume there was a problem with the 8.4.4 code.