Upgrading to 2.19.1 from 2.18, I started receiving 403 Forbidden from Fastmail CalDAV when adding events. With debug logging I can see Fastmail is objecting to:
ATTACH;VALUE=TEXT:file:
Response:
<D:responsedescription>Invalid VALUE type for property ATTACH: VALUE=TEXT</D:responsedescription>
Looking at the RFC I would tend to agree with Fastmail.
I have downgraded to 2.18 and seems fine again now.