To backup a file you must be in single user mode. Normally the program will ask you to change to single user when a back up is requested and this happens even when using RDA. If the second user does not log out properly it causes this error as well when trying to complete a backup. When in RDA you must log out for everything to work properly and not let the program time out. If the connection times out I have experienced a loss of entries as the data was not properly stored back to the cloud. There are some of these little quirks with RDA but it works well when used according to the guidelines.

There is nothing about a backup. When I access my client's shared file, the system asks me to save the file to my computer. When I attempt to do that, a message pops up saying the file is being used by another application. So I can't work on the file.

This sounds like the client has the file open. Be sure they are not leaving the file open but fully closed and then try again.