A common issue lately has been MODX Manager having a blank page, or non-functional menu, no resource tree, can't log out, nothing!
In most cases, this is all due to two settings, compress_css and compress_js. By default, they are both set to 1 (On). In most cases, regardless of the server these should be set to 0, or (Off).
This problem rears it's ugly head suddenly, after updating, and sometimes just because. It can happen on sites that functioned perfectly one minute, and the next all of a sudden the manager is broken. It happens often after updating, not only from MODX 2.2.x to 2.3.x, but within the two main versions.
MODX has attempted to solve this with a check upon installing, but from what I can tell this doesn't ever change the database settings, for any reason. Managers that suddenly become broken, pass the same checks on updating as a new installation. At the moment, this is something we have to manage for ourselves.
First of all, you won't be able to access your system settings through the manager, so you will need access to your database, usually via phpMyAdmin or whatever module your server has provided.
Access your MODX database and navigate to the system_settings table. Search (ctrl+f / cmd+f) for "compress".
Edit each setting:
Exit your database, you're done here.
Go to your site's core/cache folder. File manager in your server's control panel is the faster way, second only to SSH/CLI if available.
Delete all the files in the core/cache folder. They will repopulate. You can actually delete the cache folder itself and it will repopulate but it seems weird and I usually take the full step to only delete the files. I also always make sure that I'm deleting the core/cache path, because once I deleted the CORE folder of a site and that was a bit distressing.
Clear your browser's cache, reload the manager, log in. Things should be better.
If not, time to Reinstall MODX.