Originally from Anathema (he/him), on the official Discord Server:

Every so often we see a surge in users caught unaware by a particular difficulty related to the use of automated backup and sync services such as Dropbox, Google Drive, OneDrive, iCloud and more.

We would like to take this time to reiterate to everyone that we do not support the use of these kinds of services for storing your user data, and that doing so can and will result in corrupted database files.

Why shouldn't I use a sync service?

Storing your data via automated backup or sync services can result in database corruption in cases where the files are being read or written when the sync service operates, and may periodically replace lock files that Foundry VTT uses to prevent additional instances of Foundry VTT from accessing the same data at the same time.

We hope this helps clarify some issues for users about backup services, and prevents some user data loss in the future!

For more information and a deeper explanation including how to use these services responsibly and protect your Foundry VTT data; please see our article:

https://foundryvtt.com/article/automatic-backups/

I take this chance to encourage people to join the Official FoundryVTT Discord. I know the platform is pretty awful for discussion and especially for research purposes, but it's probably the best official channel to communicate with the devs and receive communications from them. Personally, I keep everything but the "announcements" channel muted, this way I get notified about important stuff like this article or software updated, and I'm having a good experience so far.

Have fun, stay safe,
~Scevola