I have had a similar reply from Royal Mail
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Hi Jenny,
Thank you for your email and enquiry,
Other Squarespace users have contacted us with the same issue and there is no fix as yet however it appears the redirect from Click & Drop to Squarespace isn't working correctly.
We can only suggest attempting to re connect using an alternative Internet browser and also white list URLs and IP's with any firewall or security programs Click & Drop IP addresses and URLs for security settings
Apologies for any concern this has caused.
I hope the details provided are useful, please reply or contact our Click and Drop helpdesk 0345 266 1004 if you require further help with your enquiry.
Kind regards
James
Click and Drop Team