Hmm. Made a mistake there.
A more thorough check on the laptop running Mojave 10.14.6 and Safari 13.1.1 shows the same problem: spinning gear wheel in the menu bar which then shows 'failed' after quitting Safari and the clear cache script runs.

FWIW I tried stopping Cookie6 and installed a copy of Privatus 6.1.8 to see what happened.
Again the same problem: following Safari -> quit the small gear wheel in the menu bar with clearSafariCache.sh failed.
Since the problem has started fairly recently it suggests that an update to either MacOS or to Safari is behind the problem. I also have a laptop running the last version of Mojave and that is not showing the same problem.
I should have added that permissions for Privatus are the same as those for Cookie6

@SweetP Unfortunately I think I've confused this thread, but I run as user.

The scripts folder is in ~/library/application scripts/com.sweetpproductions.CookieApp

Folder permissions are:
user (me): r/w
everyone: read only

Permissions on the Scripts are:
User (me): R/w
staff: read only
everyone: read only

Rather than start a new thread I'll just point out that I'm having the same problem but with Safari cache. I get the same error as above but it's the Safard cache script that's failing/ I've not tried the tech preview scripts but that's probably duff as well.

I've done a complete - or AFAICT full delete - of Cookie6, reinstalled via the App Store and downloaded the scripts in the link above for both Safari and SafariTP. These are placed in ~/library/application scripts/com.sweetpproductions.CookieApp (4 scripts in all)

Since the problem may relate to Sandboxing, should the scripts be somewhere in ~/library/containers/com.sweetpproductions.CookieApp?

Permissions on ~/library/application scripts/com.sweetpproductions.CookieApp are: user r/w everyone no access

They are the same for ~/library/containers/com.sweetpproductions.CookieApp user r/w everyone no access

Anyone any further ideas?

Just clocked that v2 is now available. I'm on Mojave so the automatic sync feature is out. However the import/export function although a bit of a faff does the trick.

Thanks for the update.

Please add another one for a sync feature.
In the meantime, is there a way of manually transferring session data? EG simply copy a folder across to the other machine.
For some reason the export utility dosn't work for me.

I'm using cookie 6 app store version and notice that HSTS cookies are not being deleted for non-favourites. I have removal set to delete the caches on Safari &/or Cookie quit. The scripts are in place.

The cache removal script 11/04/19 is:
usr/bin/killall -KILL nsurlstoraged
find /private/var/folders ! -name "SafariTechnologyPreview" -name "com.apple.Safari"
-exec rm -r {} ; 2>/dev/null

Not sure why there needs to be a separate script but I seem to have aquired a Safari TP cache script 28/03/19 (there's another forSafari TP history):

/usr/bin/killall -KILL nsurlstoraged
find /private/var/folders -name "SafariTechnologyPreview" -exec rm -r {} ; 2>/dev/null

No scripts are needed for Firefox

OK thanks Russell

Do we need Scripts for Firefox and if so, where weould I find them.

Thank you for the response.

I'm not sure which they are but - most likely cached cookies. Since a quit and restart Safari "apparently" eliminates all those cookies that are not set as favourites in Cookie, it sounds as though that is what is happening.  Cookie does delete them but it's not reflected in Safari until it's done a restart.  If that is the case then I think Cookie is probably working as intended.

In the past Cookie would show in red those cookies marked as tracking ones.  Now it simply lists that there are X number tracking cookies.  It would be good if the old behaviour could return.