I do not have a deep technical expertise, but I hope my experience is helpful to you.

I have installed Dropbox and Caja Dropbox on several computers from Linux Mint Software Manager in LM 19, 18.3, 18.1, 18, and 17.3, and each time it worked without any terminal commands and without needing any complex fixes or workarounds. (That is different than how it operates in Windows, where clicking the desktop icon does open the Dropbox file folder.) Clicking the Dropbox icon on the desktop does nothing. Once that is done, you go to the Dropbox folder in your Home folder to access your Dropbox files. Then, it will create a Dropbox folder inside your home folder and download your files to that. Then, you run Dropbox (or you might have to run Caja Dropbox), and it will download some more software from Dropbox and take you through the installation, which includes entering your Dropbox password in a web browser. You may need to provide your computer password in order to perform this action. I don't know whether this is cause of your problem or not, but did you install Caja Dropbox *and* Dropbox? You want both. Click the Dropbox icon in the menu bar at the top of your screen Click on the gear icon in the Notifications panel and select 'Quit Dropbox' Delete the Dropbox application from the Applications folder.

You could also look in Disks or GParted to verify the file system. One easy way to verify that is to look in File Systems in MATE System Monitor. Yes, LM 19 MATE uses ext4, unless you intentionally overrode the default and chose something else at installation.

Install dropbox on my computer