At every boot, Raspberry Digital Signage cleans up the Chromium folder for security reasons.
How can I keep my cookies and so on?
You can persist the Chromium folder in the admin interface in order for every setting, extension, cookie or anythig else you apply to the browser to be saved.
If persisting the Chromium folder (which is the preferred way of doing) it’s not sufficient for your purposes (i.e. something isn’t surviving the reboot), you can completely disable the cleanup upon boot by commenting out the cleanup lines in /rds/home/rds-user/.xinitrc.
Remove all the lines regarding: # Clear all browser settings and downloads at every cycle and # Restore Chrome settings (home folder).