natali_z,
It is likely that you have a javasciprt conflict with another plugin or in some cases with the theme you are using. I recommend that you disable your plugins one at a time until the javascript error message no longer appears. Then the folder tree may appear. If not, then there is another step you can take to solve that. So first see about eliminating the javascript error.
Alan
Checked all the plugins and active theme. Not working. What to do next?
If you can send me an email at alan@maxfoundry.com, I can see about helping you fix this on Monday.
And what to send something?
There are some things that wordpress.org does not allow us to ask our users. So for that we have to communicate outside of this forum.
You should try running Media Library Folders Reset plugin that comes with Media Library Folders. First deactivate Media Library Folders and activate Media Library Folders Reset and run the Reset Database option from the Media Library Folders Reset sub menu in the dashboard. After that, reactivate Media Library Folders. It will do a fresh scan of your media library database and no changes will be made to the files or folders on your site. Then the folder tree should load correctly.
Thank you very much. It worked.