what did I do :
1/ un-install davdroid and tasks
2/ delete tasks and contacts on the webcloud.zaclys
3/ clean the addressbook on icedove(thunderbird) as there were some duplicate, due, in my opinion, to the differences of items in lightning, zaclys and android versions of the addressbook. when some item is different, a new contact with the same name is created.
4/ because of the two messages in the log :
Unparseable date: “FREQ=YEARLY” (at offset 0)
ava.lang.IllegalArgumentException: DUE must not be < DTSTART
I clean the tasks on lightning to solve the date pb. I wonder if there could be a pb with the ‘repetition’ property of a task : possible in ‘lightning’, not in ‘zaclys’ and ‘tasks’. also, some tasks, created elsewhere than in lightning, were synced but with dates in the bad zones. not sure of what happened.
after that, no any red date for any task.
5/ sync lightning and zaclys : ok
6/ something like a ‘miracle’ happened : sync zaclys and davdroid : alleluia!
7/ for the ‘contacts’ : I don’t know the reason of the ‘404’ error as the ‘owncloud’ version of zaclys is the same (8).
8/ the sync works now, in both ways, and on the three places (pc-lightning, owncloud-zaclys and smartphone-davdroid)
thanks for the help !