Hello! Sorry for this, we used to have a related bug for 2.6, but it has already been fixed in 2.6.1. Could your please tell me the D5 version you are using now?
For this time, most cases for unable to fetch asset library are related to network issues. Please check your network speed and connection. Also, you can check if you have a software conflict for D5 or some firewall software and antivirus software caused this.