Often backend errors are caused by unescaped snippets. You could check the output of the javascript files in the browser debug console and validate them in a code-editor. Just copy the reply of the requests in the network tabs into a code-editor that can show syntax errors on javascript files. In this case it should show you the „problematic“ snippet. Afterwards you can edit the snippet in the database (s_core_snippets) and add a „“ to the unescaped character.
E.g.:
Without the esaping of ’ the javasript of the backend will break. This is only needed on backend snippets.
More things to try:
- deactivate all plugins via database (s_core_plugins) and clear the cache on the server
- re-upload all backend theme-files from the install-script (themes/backend)