A troubleshooting step to see if attachment upload repository is working or not.
Please copy the Servlet path from Admin Tab->Attachment Library->Repositories->Right click on one of the Repository (or the one with issues) View.
It should look something like this:
Please then paste that Servlet path link in a browser. You should get one of the following messages if the servlet is working
"The File download request is missing some required parameters An unspecified processing error was encountered"
Even though the message sound like an error, it is an indication that the Attachment upload Servlet is up and running.