It looks like the DNSSEC DS keys persisted across the domain transfer (good!), but once you switched over to Sonic as your Authoritative Nameservers, the domain broke, since we didn't have any corresponding keys on our side to match those at the Registrar. I've removed the DNSSEC configuration for your domain at the registrar, which seems to have solved the issue and restored your domain to normal operation.
I'll see if there's anything we can do to improve the tools our Support team have to flag up this situation in the future for closer review and/or escalation.
-- Joe M
Sonic System Operations