Edit : I find out an alternative to the broken CalDAV sync : syncevolution. It works well with my memotoo.com account (in 2025!). I successfully synchronize :
- memo
- todo
- calendar events
- address book.

The steps by step I follow :
1. install syncevolution 1.3.1 (aegis-dpkg -i syncevolution_1.3-1_armel.deb)
2. run: syncevolution --configure --sync-property "username=123456" --sync-property "password=!@#ABcd1234" memotoo
3.check (with nano...) this file : home/user/.config/syncevolution/default/peers/memotoo/config.ini

Example of required entries:
syncURL = http://sync.memotoo.com/syncml
username = 123456
password = !@#ABcd1234

You may check as well with this command:
syncevolution --print-config memotoo

4. first sync (first full sync => with slow mode) :
syncevolution --sync slow memotoo addressbook
syncevolution --sync slow memotoo calendar
syncevolution --sync slow memotoo todo
syncevolution --sync slow memotoo memo
syncevolution --status memotoo addressbook calendar memo todo

5. next sync (incremental) :
syncevolution --quiet memotoo

:-)