Is cloud syncing possible with FileVault? Today, on my professional computer, I wanted to open a personal document (PDF) directly from Dropbox website. However, it was impossible for Dropbox to show a preview of this PDF file (This file can’t be previewed). Actually, I couldn't preview any of the files stored on my Dropbox, and downloading them didn't help either.

My Dropbox is synchronized with a local folder on my Mac, on which FileVault is activated.

I have another folder on my Mac which is synchronized with a webserver, using Nextcloud's app (similar to Owncloud). When using the web interface of Nextcloud, I couldn't open the files either.

Is it possible that, with FileVault activated, an encrypted version of my files are stored on Dropbox and Nextcloud server?

If yes, is there a way to circumvent this issue?

I didn't have this problem in the past, but since I activated FileVault some weeks ago, I thought it could be the cause.
No, there's no possibility of FileVault suddenly "activating" and encrypted the contents of just one file. This is not at all how FileVault works.