As a first step I would try to restart TheBrain. If this does not help, I would recommend the following:
Logout of ALL devices that have TheBrain (Note - quitting the app does not log you out, you have to open the app and click Logout)
Once logged out, reset your password via this link: https://app.thebrain.com/forgot-password (You can re-use a password any time, so don't feel that you have to come up with a NEW password again.)
Once you reset the password, login again on your devices and try to Sync.
If this does not resolve your issue, I would appreciate it if you could please write into email@example.com for further assistance.