Cannot Save Hikashop cart module if assignment set to "all Pages"

-- HikaShop version -- : 4.0.3
-- Joomla version -- : 3.9.4
-- PHP version -- : 7.0
-- Browser(s) name and version -- : Firefox 66.02 (64 Bit)
-- Error-message(debug-mod must be tuned on) -- : None

Cannot save cart module when menu assignment is set to "All pages".
To Reproduce:

Edit module in the normal way
Try to save module (click on Save)
Set menu assignments to "All Pages"
Hit Save Button

Actual Result:
Control reverts to module listing
Module is marked as 'checked out'
No Module changes are saved

Expected Result
Module is saved
Control remains with edit module mode.

Error occurs also when the 'Save and Close' button is clicked

Environment/Logs
There is no entry in error log, caching switched off.

Template is gantry 5

Further investigation shows that this is not just a Hikashop module issue: it is occurring with other module types on my system
I therefore conclude that this is either a Joomla issue, or a problem with my particular setup
Miles