Solved-Partially
Hi, I've had the same problem on my S5 hooking up gear fit 2 for the first time. I tried quite a few things, nothing worked apart from the following;
1. Clear cache and delete data in gear fit 2 plugin under apps manager
2. Uninstall gear fit 2 plugin
3. Clear cache and delete data in gear manager under apps manager.
4. Download a previous version gear fit plugin from:
Gear Fit2 Plugin 2.2.04.16111761 APK Download by Samsung Electronics Co., Ltd. - APKMirror
5. Install gear manager from play store.
6. Uninstall gear fit plugin again and reinstall with one downloaded from website link. (Enable unknown sources, you can disable again afterwards.)
7. Disable internet connection
8. Run gear manager and let it setup the watch.
9. enable internet connection and go to gear watch faces, let it update to the latest version.
This have worked on my s5, I noticed it started with the errors again more than a week later after I sent an sms. I just followed the steps again and it was fixed until I sent another sms. So I'm doing it again tonight.
My girlfriends s5 and watch was doing the same, i used above fix and it's worked ever since.
Hope samsung fixes this bug, in the meantime try this fix and keep track of what your last action is before/if it starts to act out again. Perhaps we can narrow down the errors cause.
Cheers
Janus