Unfortunately, this is not an issue with Invision Community but rather PHP 8 and your third party plugins. You will want to ensure you're using plugins compatible with PHP 8, if you run PHP 8 on your server. You could certainly downgrade PHP to 7.4 and see if that helps resolve your issue here. However, the server error log entries we were provided point to PHP 8 incompatibilities in the third party plugin. If you want to use PHP 8 and that plugin, you will need to work with the author on, third party plugins are outside our scope of support.
At time of testing this, I only saw 1 custom plugin installed. The above information is only based on what information was tested and error log entries provided to us. If you tried other plugins, which the author says are compatible with PHP 8 and you're still having issues, you would want to disable the plugin which is not working and try with that compatible plugin only enable. If you're having issues, please let us know and we can also take a look.