How can I get back access to the backend of TYPO3 when fluidpages are causing an error on backend and frontend?

Question

I got to work on a Typo3 page and I have to say I'm at the very beginning if it comes to TYPO3.

So I wanted to fix a bug and updated the plugin "fluidpages" from version 4.1.0 (I guess) to 4.3. That cause the "Oops, an error occurred!" in the backend and frontend, so I dont have access from that side. I still have the FTP login for the website. The version of TYPO3 is 8.7.18.

The errorcode I get is that one:

Could not analyse class:
"FluidTYPO3\Flux\Configuration\ConfigurationManager" maybe not loaded or no autoloader?
Class FluidTYPO3\Flux\Configuration\ConfigurationManager does not exist

So does anyone know what I need to do?
From my research I think I need to update another plugins to make it back to work? And can I update them just by uploading them through FTP?

Thanks in advance!

Edit: After I tried to get the old version of the plugin running, I'm getting this error code from the log file:

Mon, 17 Feb 2020 22:59:56 +0100 [ALERT] request="fec23f8baf6dc" component="TYPO3.CMS.Frontend.ContentObject.Exception.ProductionExceptionHandler": Oops, an error occurred! Code: 2020021722595594039a4d - {"exception":"TYPO3\CMS\Extbase\Mvc\Exception: The default controller for extension \"Flux\" and plugin \"Page\" can not be determined. Please check for TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin() in your ext_localconf.php.

Answer 1

Have you tried dumping autoload from Install Tool? Generally, it would help for the class not loaded problem. Otherwise, you may want to check the log at typo3/temp/var/log/ and find the errors in code.

Answer 2

You can enter the Install Tool via https://example.org/typo3/install.php and start with Check for broken extensions. If that doesn't yield the extension in question, you can always manually change the PackageStates.php file and remove the extension you want to unload. Afterwards you should flush all caches through the Install Tool.

In the long term you should consider a few things:

1. Use a local development environment which mirrors your production environment. You can safely perform all steps without any risk here before deploying.
2. Use a deployment strategy/tool which allows for rollbacks. This allows you to quickly revert to a working version of your instance in case all other checks have failed. A simple but flexible solution is Deployer, you'll need SSH for this though.