SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway

There are a few things to note here:

• "Network is unreachable" seems to indicate an issue with the network connection, but the connection appears to then actually work anyway.
• "unable to get local issuer certificate (20), continuing anyway" seems to indicate that the client wasn't able to get the relevant certificate from its local store, but ignored this issue. (Do you have certificate verification disabled? This isn't recommended, as a matter of security.) The Dropbox API content servers are being served with valid certificates though, so you should make sure your system trusts that and enable verification.
• You seem to be using a "Content-Type" of "multipart/form-data...", which isn't supported by the Dropbox API. The /2/files/upload endpoint expects a request of type "application/octet-stream".
• You seem to be getting a 400 error back. You should print out the response body itself, as it should contain a more useful error. (It may be related to the incorrect "Content-Type" noted above.)
• The "Failed writing body" seems to indicate that curl wasn't able to write the body out for some reason. I can't offer much support for that though as it seems to be related to curl itself.