Ahah i solved my issue while writing this post. So i post it anyway with the solution, in case it can be useful to someone else.
I have a Nextcloud in which are stored my contacts and calendars. One of the calendars is the “Contacts Birthdays” automatically generated by Nextcloud.
I’m using Davx5 with Etar (calendar app) and Simple Contacts Pro to sync with my android.
Looking at Etar yesterday, i saw that today was gonna be one of my friends birthday, thanks to an event from the “Contacts birthdays” calendar. And i told to myself “hey, let’s add a reminder to that event so that i don’t forget tomorrow”.
Since that moment, at each sync i have an error notification on my phone saying “Contacts Birthdays - Server error HTTP - HTTP 404” whereas i can access that calendar without any problem in my Nextcloud.
I suppose that the error comes from the fact that i tried to modify an event from that calendar which is a read-only calendar (because generated by the Calendar App of Nextcloud using the Contacts App).
Aaand so i solved it :
In the Calendar App of the Nextcloud, i disabled that calendar in the “Settings and import” dropdown menu in the bottom left of the screen. Then in Davx5 i synced the Calendar list, then reactived the calendar in Nextcloud and synced the calendars list in Davx5 and voilà !