Weird syncing issues with HTC One M9

[AC Question]

A few months ago, I started noticing a string of seamingly unrelated errors with my HTC One M9 that I just seem unable to fix.

A few of the symptoms:
- My work calendar (Google GSuite account) doesn't want to sync
- Dropbox doesn't want to offline sync files
- When uploading a photo to Slack, I get the error that the upload didn't succeed ('likely because we were not able to open [the file].')
- Whatsapp sync would not run unless I had the app running in the foreground
- Google Photos would not sync unless I had the app running in the foreground
- Google Play would not download updates unless I had the app running in the foreground

Since all these issues seemed to involve syncing with external services, I tried resetting some application settings that would inhibit background syncing and tried resetting my data and accounts for apps such as calendar, etc. but nothing seemed to work.

I felt like doing something drastic, so a few days ago, I completed reset my phone to factory settings, manually re-downloading only the most crucial apps and it seemed to work fine for a few days, but then, the problem resurfaced and now I'm looking for ways to determine the root cause.

Any ideas?

 

[B. Diddy]

Welcome to Android Central! Does this happen on both wi-fi and mobile data?

Just to be sure -- the Restrict Background Data setting is turned off in Settings>Data Usage​, right?