I'm having the same Dropbox issue on my Windows 11 pro (21H2) PC with Enpass 6.8.0 (1059). I have to re-auth with Dropbox at least once a day on this PC. No issues with my iOS and Android devices.
I tried removing the sync from all devices as suggested above, but after resetting everything the problem persists on my Windows 11 PC.
I also tried uninstalling Enpass and re-installing using both direct download from Enpass website and Microsoft Store version. No luck. I've never had this problem before so clearly it's a bug introduced in 6.8 for some OSes.
Look forward to getting a fix - moving my Enpass db to another cloud service for synching would be a last resort option for me.
UPDATE: This morning I had to re-authenticate Dropbox sync in the enpass app on my iPhone, which has Enpass version 6.8.0 (625) installed. So contrary to my comment above iOS devices are affected.
My Android phone has recently updated itself to Enpass 188.8.131.528 and seems fine. Is 6.8.1 the fix?
UPDATE 2: No, 6.8.1 on Android isn't a fix - I just had to re-auth on my Android device (a Pixel 4a with Android 12).