The Dropbox desktop app on my computer is giving me an error when I try to sign in

Heya Jen -- 

When it comes to Dropbox and Proxies theres two main things to keep in mind: 

1. If you don't have a proxy or you have a proxy configured in your OS - you don't have to take any action in Dropbox. Default proxy settings in Dropbox will just route your Dropbox app traffic based on whatever your OS is configured to do with it's connection. If you have a proxy in place in Windows, Dropbox will route it's traffic the same as the rest of your computer (through the proxy). 

2. Alternatively, if you want to ONLY have Dropbox traffic routed through your proxy, then you can configure your proxy settings into the Dropbox desktop application specifically and it will only affect your Dropbox traffic. The rest of your computer will route traffic based on what your OS is configured for. 

I'm not familiar with Trend Micro in particular -- it's possible it's doing something to affect your device's traffic as a whole, but I would defer to their support team for particulars on their service.

From what you described, if setting Dropbox as 'no proxy' allows traffic through normally - that makes me think your device itself does have a proxy enabled (even if it's not immediately apparent from your Windows settings), and this configuration is allowing Dropbox to circumvent your device's configured proxy to connect directly. You may want to explore this further with your provider to better understand their app.

Hope this helps!