That UnauthorizedAccessException error translates to something like Access to the path ‘agsXMPP.dll’ is denied. and the rest looks like it’s trying to delete a directory. If you have a timestamp on that, maybe you can tell if the error happens right around when “Install” is attempted – and since you tried many times maybe you have many?
Downgrading / reverting to a lower version talks about where your “updates” folder would be, and sometimes an install means a deletion of an old update (which might be what you’re seeing), however I think it keeps one back, so 2.0.3.3 would try to make 2.0.4.3, would keep the older before 2.0.3.3, and would remove the even older one. Maybe someone else can think of why else Duplicati would be trying to delete this. If you want to see the activity directly (instead of listening to me speculate) you could install Sysinternals Process Monitor to monitor the file activity around agsXMPP.dll, and see the folder. Once you find it, you can try moving it out of the updates folder manually (as the How-To mentions) and that might save Duplicati from needing to delete it, however the deletion is supposed to be possible. Are you doing any XMPP notifications that might be holding the file open? Sysinternals Process Explorer (or Handle, if you prefer command line) can help find open files. I’m not even sure an open file would throw THAT error. I’m finding some reports that a read-only file will do so.
Or maybe the browser theory will work. Also, note that the timing observation I began with may support/refute my thinking about what that delete is about.