The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
This is a standard message when things don't get through a server.
It is not necessarily literally true. And the message may not be generated
by a server. Don't know what you can do other than contact your ISP about the problem.
It might help to go through the upload process through another browser. Sometimes the browser will prevent things getting through.
Also, do you have a firewall or are you using Mircrosoft defender. Sometimes anti-virus and firewalls AI make unasked decisions to block certain things and you have then to uncheck their list of permissions.
A quick solution. Send your intended uploads to somebody else in Admin in CB+ and get them to do the upload.
I would be surprised if they don't get through that way.
Cheers!