Thank you for contacting us. This issue started happening just after the Android 12 release, and we're not sure yet what's causing it. A different user has figured out that deleting the Dropbox app fix the problem with the error message. The problem with this work-around is that if you do it, all files and folder that are favorited and stored locally on your device will be removed. When you reinstall the Dropbox app, you can then favorite the files again, but do be aware they'll be removed in the process. The alternative to deleting the Dropbox app would be to use mSecure Cloud syncing instead at least until mSecure 6 is released.
@Olaf From our understanding after helping customers with through this error message, if you uninstall the Dropbox app, then go through the linking process in Dropbox again, mSecure will have no problems linking to your account. It's the only thing that has fixed the problem with all of the customers that have encountered the issue. Did you delete the Dropbox app and then try to set up the Dropbox sync feature in mSecure while the Dropbox app was no longer installed?