The ODK Briefcase shows "Fail" on Pull Status when pulling data from the
server since two weeks, before that it was always successful. It shows
"Fail" after fetching instances until "....fetching instance 682 ...
retrieving next chunk of instances from server...
FAILED."
What is the error that you're seeing on Aggregate? It sounds like you may have received submissions that are corrupt in some way. The more information you can provide on the errors you see and the kind of issues that there may be with the submissions, the more likely someone will be able to help!
For the submissions not retrieved, there might be some corrupt data on your server. What happens when you export the same form as a CSV directly inside the Aggregate UI? Does it work then?