Revisited this issue and posting here in case others run into the problem.
The error was a red herring and threw me down the wrong path. I figured out that it was an nginx proxy config issue that was preventing the dropbox integration from fully working. Weird.
I have nginx in front of companion. Here’s the post that helped me sort out the config:
The OP was having an issue, but their config is correct for nginx+companion.
No more error!