The CRXDE/Package Manager was accessible by login through crx/explorer(http://localhost:4503/crx/explorer/index.jsp) or through system/console(http://localhost:4503/system/console) but not directly.
After analysis, identified the issue can happen in one of the below scenarios
1. Read access for the anonymous user to read own user node has removed
2. Anonymous user deleted by mistake
The actual root cause for our issue was first scenario - Read access for the anonymous user to read own user node has removed, the anonymous user did not have the permission to access own user node - by default AEM enables the read permission to the anonymous user to access its own user node.
The issue was caused after accidentally removing the read access to anonymous user to read own user node.