we cannot download any files from Server(Asset Manager)

Hello,
We are a church located in Hong Kong. Recently we used your free website service of “Atua“ from WordPress. When we used the “Add File” of “Download” at the left side, we came across some problems:
1) I clicked the “select from servers” at the lower left corner, but it didn’t show the files stored in “download-manager-files” of “Asset Manager “. “Home-undefined ” appeared on the left-hand side and “Error! Session expired. Try refreshing page” appeared on the right side. Then I following the instruction, and tried to refresh the page but it didn’t work.
2) I entered again to “Add Files”. The website appears “新增標題” which is “Add Files” only, without showing “Select From Server” function.
Therefore, we cannot download any files from Server. Appreciate your help teaching us how to resolve it. Await your reply!