I find out an alternative to the broken CalDAV sync : syncevolution. It works well with my memotoo.com account (in 2025!). I successfully synchronize :
- memos
- todos
- calendar events
- addresses 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...) home/user/.config/syncevolution/default/peers/memotoo/config.ini.
Examples 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 (slow mode required the very first time) :
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

It works well so far. However how to run it automatically with a defined frequency ?