To troubleshoot file tree problems, try the following
1)Backup your WP-database and WP Directory (if needed)
2)Disable all plugins
3)Go to Dashboard -> Updates ->Reinstall-Now (This will reinstall WP core files)
4)Now test out WP-Filebase File Browser and it will load.
5)Enable one plugin at a time and test it out
I had similar problem.The file browser did not work and also when i am adding a file, the post ID did not work.I had some Ajax errors. Just disabling all plugins except WP-filebase did not solve for me initially and this made me believe the problem was not caused by other plugins. Only after disabling all plugins and then reinstalling WP core files (step 3 as above) it solved my problem. Since i did not believe the problem was caused by other plugins, i reactivated them and the problem came back a day later after i activated the plugins as if there was some cron job running once a day!
After reinstalling core files, the File browser worked well. So it means, WP-Filebase works well as long as WP files are in original state. It also means another plugin should be the culprit. Eventually i found my culprit! The plugin WP-Better Minify was the cause. Disable and uninstall the plugin and reinstall WP core files once more solved the problem once an for all...