Upgrade from 1.7.6.2 to 1.7.8.8 - back office login page not working

[roboy]

I just tried to upgrade from PS 1.7.6.2 to 1.7.8.8 using the latest version of the 1-click upgrade module. I went through the process without issues and afterwards the upgrade appeared to have been successfull with green ticks and everything.

Luckily I followed best practices and performed the upgrade on a sandbox installation mirroring my live shop. When visiting the back office page after the upgrade I get the following error message:

Quote

Oops... looks like an unexpected error occurred

You should use `SymfonyContainer::getInstance()` instead of `ContainerBuilder::getContainer('admin')`

[PrestaShopBundle\Exception\ServiceContainerException 0]

I did follow the checklist given after upgrade and cleared browser cache and cookies, but that has had no effect.

I would be very grateful for any help or pieces of information that could lead me on the right path to resolving this issue.

By the way, the server is running PHP v7.3 in case that is important.

 

[roboy]

So, just to keep you updated I made a second attempt to upgrade, this time disabling all non-native modules (I hadn't done that the first time). It made absolutely no difference and ended with the same error message, so I suppose this narrowed down the issue to being related to the Prestashop core or native modules.