Ian Garmaise writes:

> When I archive one subtree (C-c $), the first one succeeds.
> The second archive operation fails with a permission denied error as shown
> in the messages buffer:
[...]
> Noticed this yesterday.  Updated org and all packages, then tried it again
> today, was able to reproduce it easily

Hmm, was that an update from 9.3.* or earlier?  9.4 came with a new
option org-archive-subtree-save-file-p.  With the default value, the
file is saved when archiving from an Org buffer but not the agenda.
Before 9.4 [*], the file was never saved, so you could set
org-archive-subtree-save-file-p to nil to restore the pre-9.4 behavior.

That should sidestep the issue, though I don't know why you're hitting.
I'm guessing you only see it with dropbox files?

[*] Going farther back, the behavior was to always save.  That changed
    in 9.1.4 63f6e851b (Do not save target buffer after archiving
    subtree, 2017-11-25).