For the specific message you posted, IPS said in a different thread that they increased the level of logging verbosity as a result of the change with PHP8 compatibility checking for 3rd party apps. It was mentioned this warning can be ignored and will be suppressed with a later update.
They wanted more logging data initially for any potential troubleshooting.
But that error is 100 percent unrelated to not being able to access admin CP. Have you tried disabling 3rd party apps? If you can’t access the ACP at all, use recovery mode to disable everything and then turn them on one by one till you find the culprit.