tomboy

Small app to organize personal notes.
Notes can be linked to notes.
Notes can be synchronized.
URL: https://wiki.gnome.org/Apps/Tomboy
Thinlinc: 
Applications > Accessories > Tomboy Notes
Synchronize
Install and configure any cloud storage like drive.math.uzh.ch, drive.switch.ch, Dropbox.
Create a directory like 
~/Nextcloud/Tomboy (Nextcloud is the personal cloud storage directory).
Open 
Preferences > Synchronization
Configure:
Service: 
Local Folder
Folder Path: 
~/Nextcloud/Tomboy
Set: 
Automatically Sync in Background Every 5 Minutes
Problems
Q: No sync at all:
A: Check that the Nextcloud Client is fine. Quit the client and restart it (on all computers).
Q: Outdated notes:
A: Have you waited 5 + 5 minutes?
A: Nextcloud might have a problem. Check 'Activity' in the nextcloud client (on all computer: a changed file should appear).