At your own risk, you may try this package :
https://git.sailfishos.org/dcaliste/buteo-sync-plugin-caldav/blob/upgrade-4.0.1/buteo-sync-plugin-caldav-0.1.75-1.armv7hl.rpm
- To install it :
devel-su pkcon install-local buteo-sync-plugin-caldav-0.1.75-1.armv7hl.rpm - to return to the version by Jolla :
devel-su pkcon install buteo-sync-plugin-caldav
This is a compilation of the version available in 4.0.1.48 plus the commit I spoke about some messages above.