in May 2018 we have announced "old clients" (via email) that the software will not be able to connect anymore due to certificate changes.
"Deleting" here means, removing the device from the account.
But to be clear, no data was or is deleted.
Now, to fix this: please contact us at firstname.lastname@example.org, referring to the post here.
I will take care of your issue and help you to regain access to your files.